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.
Fixes attempts to update a service integration which had `can_test?`
set to true but validations were causing the "Test and save changes"
button to return "Something went wrong on our end."
Removes references to index action left from 0af99433143727088b6a0a1b2163751c05d80ce6