Restrict mirroring to admins

Description

As currently implemented anyone with owner or master role for a project can set up a mirrored repository. In some environments mirroring open source projects into their instance is problematic, and needs to be carefully reviewed and approved.

Although it will still be possible to manually bypass this by creating an empty project, an manually mirroring via a local repository with two remotes, preventing the automated mirroring is sufficient.

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

Proposal

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

Mirroring enabled	Mirroring disabled