Add i18n-tasks gem

[gravitystorm]

While refactoring some translations strings I found one containing raw html without the _html suffix - so I knew that that it hadn't been used since rails 4 was released! I tracked it down and the use of the string was removed in 2013, so it's worth finding if there are more strings that might be completely unused but still creating work for our translators.

I found some more by using https://github.com/glebm/i18n-tasks/ which does some static analysis to figure out what is in use. It will need some configuration, particularly to avoid false positives in keys used by activerecord and error handling, but it's still very useful.

[gravitystorm]

The gem is now added, but it needs further work before we're using any of the tests in CI. The first target is to remove the remaining unused translations, and update the configuration to ignore the remaining false positives.