Externalize strings for translation in service models

Summary

The service models in app/models/project_services contain a lot of UI strings, not all of which are prepared for translations.

We should clean this up by using s_() with the service class as namespace (see e.g. https://gitlab.com/gitlab-org/gitlab/blob/8fb5f42b7f683815c8929cf4a1f05238132156a2/app/models/project_services/jira_service.rb#L124-132).

There are also more strings in lib/api/helpers/integrations_helpers.rb / ee/lib/ee/api/helpers/integrations_helpers.rb.

Background

The following discussion from !46564 (merged) should be addressed:

• @toupeira started a discussion: (+3 comments)

  suggestion: We should fix the casing for GitLab:

      'Trace your GitLab pipelines with Datadog'

  It would also be nice to support translations (same for the strings in fields below):

      s_('DatadogService|Trace your GitLab pipelines with Datadog')

  Although I see translation support is pretty spotty in existing services too, so maybe it would be easier if we created a follow-up issue to fix all those at once 🙂 WDYT @Andysoiron?

  @AdrianLC if you do want to go ahead with this you'll have to take special care for the HTML in the password help text (https://docs.gitlab.com/ee/development/i18n/externalization.html#html), and update the PO files with the gettext:regenerate Rake task (https://docs.gitlab.com/ee/development/i18n/externalization.html#updating-the-po-files-with-the-new-content).