Adds admin application setting to globably disable push mirrors.

What does this MR do?

Restricts the usage of push mirrors to admins.

Are there points in the code the reviewer needs to double check?

Do I need to add API support for this. I do not think so.

Do I need to create a administration doc explaining what this checkbox does or is it fine to link to the push mirroring feature?

Screenshots (if relevant)

Admin push mirror setting:

View from user with Master permissions in project with the admin setting turned on:

View from Admin in project with the admin setting turned on:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #1144 (closed)