Add users allowlist to ApplicationRateLimiter

What does this MR do?

Related to #320965

Adds the option to set a list of usernames that will be excluded from the rate limit on notes creation via web, API and GraphQL.

Migration 20210210210232_add_notes_create_limit_allowlist_to_application_settings

db:migrate:up VERSION=20210210210232

== 20210210210232 AddNotesCreateLimitAllowlistToApplicationSettings: migrating 
-- add_column(:application_settings, :notes_create_limit_allowlist, :text, {:array=>true, :default=>[], :null=>false})
   -> 0.0054s
== 20210210210232 AddNotesCreateLimitAllowlistToApplicationSettings: migrated (0.0055s)  

db:migrate:down VERSION=20210210210232

== 20210210210232 AddNotesCreateLimitAllowlistToApplicationSettings: reverting 
-- remove_column(:application_settings, :notes_create_limit_allowlist, :text, {:array=>true, :default=>[], :null=>false})
   -> 0.0058s
== 20210210210232 AddNotesCreateLimitAllowlistToApplicationSettings: reverted (0.0072s) 

Screenshots (strongly suggested)

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