Signed-off-by: Rémy Coutable <remy@rymai.me>

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.

Improvements and refactorings were made while adding role based permissions for protected tags to EE. This doesn’t backport the feature, but should improve code quality and minimize divergence.

Added changelog

Fixes ProtectedBranches#create flash errors bug due to
typo in 'flash[:alert] = @protected_branches.errors'