Add project setting to toggle job token scope

What does this MR do?

Related to #329550 (closed)

This MR is a follow-up from !62733 (merged) where we introduced a scope of action for a CI_JOB_TOKEN to limit the projects it can access to.

In this MR we are adding a project setting to enable/disable this feature.

We introduce this setting as "disabled" in the database so it won't apply to existing projects but we set a default value true in the model so that new projects will have it enabled by default.

The change is applied behind a feature flag ci_scoped_job_token which was introduced in the previous MR.

Database migration

$ rails db:migrate:redo          
== 20210607154719 AddJobTokenScopeEnabledToCiCdSettings: reverting ============
-- remove_column(:project_ci_cd_settings, :job_token_scope_enabled)
   -> 0.0033s
== 20210607154719 AddJobTokenScopeEnabledToCiCdSettings: reverted (0.0132s) ===

== 20210607154719 AddJobTokenScopeEnabledToCiCdSettings: migrating ============
-- add_column(:project_ci_cd_settings, :job_token_scope_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0038s
== 20210607154719 AddJobTokenScopeEnabledToCiCdSettings: migrated (0.0098s) ===

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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