Improve diff lines count efficiency

Enables frozen string for the following:

* lib/gitlab/fogbugz_import/**/*.rb
* lib/gitlab/gfm/**/*.rb
* lib/gitlab/git/**/*.rb
* lib/gitlab/gitaly_client/**/*.rb
* lib/gitlab/gitlab_import/**/*.rb
* lib/gitlab/google_code_import/**/*.rb
* lib/gitlab/gpg/**/*.rb
* lib/gitlab/grape_logging/**/*.rb
* lib/gitlab/graphql/**/*.rb
* lib/gitlab/graphs/**/*.rb
* lib/gitlab/hashed_storage/**/*.rb
* lib/gitlab/health_checks/**/*.rb

Partially address gitlab-org/gitlab-ce#47424.

- Creates a new column to hold the single patch limit value on
application_settings
- Allows updating this value through the application_settings API
- Calculates single diff patch collapsing limit based on
diff_max_patch_bytes column
- Updates diff limit documentation
- Adds documentation (with warning) as of how one can update this limit

We used to apply this limitation on GitLab when using Rugged. Now that
we've shifted to Gitaly, this parameter needs to be sent via a RPC
request. Currently this value is hardcoded on Gitaly.