Add Files API throttling to application settings

What does this MR do?

Contributes to #335075

Roadmap

1. Database migration MR <== This MR
2. Throttling configuration form in Admin Area MR: !68560 (merged)
3. Rack attack configuration MR: !68561 (merged)
4. Documentation update: !68645 (merged)

Why

We want to have an option to rate limit Files API requests.

Proposal

Extend application_settings to support rate-limit configuration for Files API.

Set limit to unauthenticated Files API requests to max 125 req. in 15 seconds.

Set limit to authenticated Files API requests to max 500 req. in 15 seconds.

Disable Files API rate-limiting by default

New rate-limit settings

Unauthorized Files API requests	Authorized Files API requests
~8 req/s	~33 req/s
500 req/min	2000 req/min

Request per minute limitations match default API rate-limits: https://docs.gitlab.com/ee/user/gitlab_com/index.html#gitlabcom-specific-rate-limits. But the new 15 seconds window protects the Files API from request bursts.

Current MR

• Adds migration to extend application_settings table
• Allows to update throttling settings

Database

Migrate

== 20210819120243 AddThrottleFilesApiColumns: migrating =======================
-- add_column(:application_settings, :throttle_unauthenticated_files_api_requests_per_period, :integer, {:default=>125, :null=>false})
   -> 0.0060s
-- add_column(:application_settings, :throttle_unauthenticated_files_api_period_in_seconds, :integer, {:default=>15, :null=>false})
   -> 0.0016s
-- add_column(:application_settings, :throttle_authenticated_files_api_requests_per_period, :integer, {:default=>500, :null=>false})
   -> 0.0021s
-- add_column(:application_settings, :throttle_authenticated_files_api_period_in_seconds, :integer, {:default=>15, :null=>false})
   -> 0.0015s
-- add_column(:application_settings, :throttle_unauthenticated_files_api_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0023s
-- add_column(:application_settings, :throttle_authenticated_files_api_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0014s
== 20210819120243 AddThrottleFilesApiColumns: migrated (0.0152s) ==============

Rollback

== 20210819120243 AddThrottleFilesApiColumns: reverting =======================
-- remove_column(:application_settings, :throttle_authenticated_files_api_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0053s
-- remove_column(:application_settings, :throttle_unauthenticated_files_api_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0012s
-- remove_column(:application_settings, :throttle_authenticated_files_api_period_in_seconds, :integer, {:default=>15, :null=>false})
   -> 0.0012s
-- remove_column(:application_settings, :throttle_authenticated_files_api_requests_per_period, :integer, {:default=>500, :null=>false})
   -> 0.0018s
-- remove_column(:application_settings, :throttle_unauthenticated_files_api_period_in_seconds, :integer, {:default=>15, :null=>false})
   -> 0.0012s
-- remove_column(:application_settings, :throttle_unauthenticated_files_api_requests_per_period, :integer, {:default=>125, :null=>false})
   -> 0.0013s
== 20210819120243 AddThrottleFilesApiColumns: reverted (0.0161s) ==============

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team