Add toggle to remove Analytics left nav item

What does this MR do?

Issue: #224700 (closed)

Allow to remove analytics nav item from the project's sidebar.

NOTE: This MR was originally implemented by @vyaklushin , but @cwoolley-gitlab is handling the review/merge phase, due to the reorg of the editor groups.

Screenshots (strongly suggested)

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Tasks

• Implement the feature (already completed by @vyaklushin when @cwoolley-gitlab took over the MR)
• Figure out why the Capybara integration spec started being flaky on CI

Database

Migration output

➜  gitlab git:(224700_add_toggle_to_remove_analytics_item) rake db:migrate:up VERSION=20201021155606
== 20201021155606 AddAnalyticsAccessLevelToProjectFeatures: migrating =========
-- add_column(:project_features, :analytics_access_level, :integer, {:default=>20, :null=>false})
   -> 0.0016s
== 20201021155606 AddAnalyticsAccessLevelToProjectFeatures: migrated (0.0044s) 

➜  gitlab git:(224700_add_toggle_to_remove_analytics_item) ✗ rake db:migrate:down VERSION=20201021155606
== 20201021155606 AddAnalyticsAccessLevelToProjectFeatures: reverting =========
-- remove_column(:project_features, :analytics_access_level, :integer)
   -> 0.0012s
== 20201021155606 AddAnalyticsAccessLevelToProjectFeatures: reverted (0.0043s) 

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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