Externalize strings for translation in service models

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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:

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