Improve CI for external repo with configurable maximum mirroring frequency on self-hosted

What does this MR do?

This changes the pull mirroring interval that was previously 5 minutes to a configurable value.

Database Review Data

▶ rake db:migrate
== 20201204215353 AddPullMirrorIntervalToPlanLimits: migrating ================
-- add_column(:plan_limits, :pull_mirror_interval_seconds, :integer, {:default=>300, :null=>false})
   -> 0.0089s
== 20201204215353 AddPullMirrorIntervalToPlanLimits: migrated (0.0090s) =======

   (2.3ms)  ALTER TABLE "plan_limits" ADD "pull_mirror_interval_seconds" integer DEFAULT 300 NOT NULL

database lab:

exec ALTER TABLE "plan_limits" ADD "pull_mirror_interval_seconds" integer DEFAULT 300 NOT NULL
Session: 1417
The query has been executed. Duration: 76.000 ms (edited) 

▶ rake db:rollback
== 20201204215353 AddPullMirrorIntervalToPlanLimits: reverting ================
-- remove_column(:plan_limits, :pull_mirror_interval_seconds, :integer, {:default=>300, :null=>false})
   -> 0.0131s
== 20201204215353 AddPullMirrorIntervalToPlanLimits: reverted (0.0152s) =======

(1.3ms)  ALTER TABLE "plan_limits" DROP COLUMN "pull_mirror_interval_seconds"

database lab:

exec ALTER TABLE "plan_limits" DROP COLUMN "pull_mirror_interval_seconds"
Session: 1417
The query has been executed. Duration: 14.000 ms (edited) 

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

Related to #237891 (closed)