* Run rubocop --autocorrect on app/, config/ and lib/, also manually fix some remaining style issues
* Run rubocop --autocorrect-all on db/
* Run rubocop --autocorrect-all on `spec/` and fix remaining issues
- 500.html generated with admin-set default locale if set
- Error page `<title>` includes Mastodon site title
- 500 title changed to "This page is not
correct" (ref: <https://www.youtube.com/watch?v=2VCAP_seh1A>)
- 500 content appended with "on our end" to make clear it's
not user's fault