Allow admins to restrict push mirroring by project

Description

As currently implemented anyone can set up a mirrored repository. This includes push mirroring which allows code to be mirrored to arbitrary places. In some corporate environments this needs to be controlled by limiting to certain groups.

Feedback from customer is

the ability to set up an external mirror is available to anyone (with write permission). We would like to restrict this functionality to a particular group of users. Is there anyway we can do this?

Requested by: https://gitlab.my.salesforce.com/00161000004yLEy

Proposal

• Instance admins may disable push mirror using from Applications Settings (in /admin/application_settings): [ ] Disable push repository mirroring
• If disabled, only instance admins may configure push mirroring

Previous description

### Description

• Add a new setting in the Settings (in /admin/application_settings), as shown below

• The dropdown allows to search group names.
• When activated, the following are restricted to given groups:
  • access to <repository_name>/mirror
  • visibility of mirror repository feature during project creation and editing