Allow project masters to toggle write permission for global shared deploy keys

Overview

Management of global shared deploy keys was improved (relatively) recently and write permission for these keys is now managed on a per-project basis (it used to have only one global value set by the administrator). But you still need to be an administrator to change the per-project write permission, even though this setting now have the same level of granularity as enabling a global key for a project, which is managed by the project masters.

Proposal

Change the privilege level needed to toggle write permission to a project for a global shared deploy keys from administrator to project master (same as the one needed to enable a global shared deploy keys on a project).