Move locale selection from form into translation index.

By selecting a locale before inputting a translation we can reduce
the number of views, remove duplication in controller, and fix the
bug in pivotal[1] where the select input has to be disabled when
editing an existing translation.

[1] https://www.pivotaltracker.com/story/show/44462687
Merge pull request #223 from alphagov/unusable-selector-on-translation-form

Move locale selection from form into translation index.

https://www.pivotaltracker.com/story/show/44462687