Add users allow-list for issues rate limit

What does this MR do?

#214789

Adds the ability to exclude users from the rate limit that applies to issues creation requests.

Screenshots

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

Migration outputs

Add column issues_create_limit_users_allowlist to table application_settings (20200430112800).

UP

bin/rails db:migrate:up VERSION=20200430112800
== 20200430112800 AddIssuesCreateLimitUsersAllowlistToApplicationSettings: migrating 
-- add_column(:application_settings, :issues_create_limit_users_allowlist, :string, {:array=>true, :limit=>255, :default=>[]})
   -> 0.0023s
== 20200430112800 AddIssuesCreateLimitUsersAllowlistToApplicationSettings: migrated (0.0024s)

DOWN

bin/rails db:migrate:down VERSION=20200430112800
== 20200430112800 AddIssuesCreateLimitUsersAllowlistToApplicationSettings: reverting 
-- remove_column(:application_settings, :issues_create_limit_users_allowlist, :string, {:array=>true, :limit=>255, :default=>[]})
   -> 0.0014s
== 20200430112800 AddIssuesCreateLimitUsersAllowlistToApplicationSettings: reverted (0.0026s)