WIP: Add rubocop-i18n

What does this MR do?

Adds rubocop-i18n (see https://github.com/puppetlabs/rubocop-i18n) to our codebase to detect untranslated strings in Ruby files.

Except for GetText/DecorateFunctionMessage (https://github.com/puppetlabs/rubocop-i18n#gettextdecoratefunctionmessage) the following cops were enabled:

• GetText/DecorateString (https://github.com/puppetlabs/rubocop-i18n#gettextdecoratestring)
• GetText/DecorateStringFormattingUsingInterpolation (https://github.com/puppetlabs/rubocop-i18n#gettextdecoratefunctionmessage)
• GetText/DecorateStringFormattingUsingPercent (https://github.com/puppetlabs/rubocop-i18n#gettextdecoratestringformattingusingpercent)

The following MRs were opened to externalize those strings:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26845
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26420
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26357
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27106
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27181
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27207
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27220
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10989
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27369
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27390
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27410

EE Port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10987

Missed cases

In the following cases rubocop-i18n wasn't able to detect offenses:

• Strings without a trailing ., ! or ?
  • See https://github.com/puppetlabs/rubocop-i18n#ignored
• Individual words
  • See https://github.com/puppetlabs/rubocop-i18n#ignored
  • This affects for example title and placeholders attributes in models and services (e.g. pipelines_email_service.rb or jira_service.rb)
• Strings that that don't start with a capital letter.
  • See https://github.com/puppetlabs/rubocop-i18n#ignored
• Concatenated strings, e.g. in split sentences like in app/helpers/visibility_level_helper.rb or milestone_time_for in app/helpers/milestones_helper.rb

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57968

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer