i18n linters - Rubocop

Part of https://gitlab.com/gitlab-org/manage/issues/20

We can lint for unwrapped strings (in simple/common cases) in ruby files using rubocop.

We should autofix as many cases using the linter as we can and this should be split across many MRs to allow for easy manual review.

Started in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24737 using https://github.com/puppetlabs/rubocop-i18n.

These cops report a large number of offenses that should be reviewed manually to find clear edge cases. One of the rules has autofix functionality but its impact is very limited. I have forked it and written broader acting autofix functionality but its changes are yet to be verified.