Add compliance merge request approval settings

What does this MR do?

This MR allows administrators to scope merge request approval settings to specific compliance frameworks.

Feature flag

This feature is under a feature flag: admin_compliance_merge_request_approval_settings

Design notes

The process flow presented in this MR is as followed:

graph LR
  a[PUT admin/application_settings] --> b[ApplicationSettingsController] --> c[ApplicationSettings] --> d[application_setttings] 

More technical discussion can be found here #219359 (closed)

Screenshots

Feature disabled	Feature enabled

Database changes

This MR adds a new column to application_settings

Column	Type	Nullable	Default
compliance_frameworks	smallint[]		'{}'::smallint[]

rake db:migrate:redo VERSION=20200613104045
== 20200613104045 AddComplianceFrameworksToApplicationSettings: reverting =====
-- remove_column(:application_settings, :compliance_frameworks)
   -> 0.0016s
== 20200613104045 AddComplianceFrameworksToApplicationSettings: reverted (0.0067s) 

== 20200613104045 AddComplianceFrameworksToApplicationSettings: migrating =====
-- add_column(:application_settings, :compliance_frameworks, :integer, {:limit=>2, :array=>true, :default=>[]})
   -> 0.0027s
== 20200613104045 AddComplianceFrameworksToApplicationSettings: migrated (0.0054s)

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