It is important to destroy data related to repo mirrors when they are
disabled.
Use `_destroy` nested attribute instead of `enabled` for push mirrors.
Call `remove_import_data` after saving a project if its pull mirror is
disabled.
Enables frozen string for some vestigial files as
well as the following:
* app/controllers/projects/**/*.rb
* app/controllers/sherlock/**/*.rb
* app/controllers/snippets/**/*.rb
* app/controllers/users/**/*.rb
Partially addresses #47424.
Luke Bennett