i18n linters - haml_lint

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

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

• 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.

• We should use danger to enforce externalization in the changes of a branch going forward.

• We should generate a list of cases that can't be autofixed to be worked through manually or to have autofix functionality written for them.

I am yet to investigate any existing solutions.