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

Enables frozen string for the following:

* app/controllers/*.rb
* app/controllers/admin/**/*.rb
* app/controllers/boards/**/*.rb
* app/controllers/ci/**/*.rb
* app/controllers/concerns/**/*.rb

Partially addresses #47424.

And use .rubocop.yml to exclude paths we don't care,
rather than using the cop itself to exclude.

MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9250/

See https://gitlab.com/madlittlemods/gl-mock-ci-service

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

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5620#note_13539571

Closes #20115

Also add Service#event_names so that we don't have to hard code all
event names in ServiceParams. Note that I don't know why we want to
call plural on issue_event and merge_request_event so that we still
need to hard code them for issues_event and merge_requests_event.
See app/helpers/services_helper.rb for those special rules.