User REST API to fetch source and translation text

There is still plenty to do with caching phrases - I will handle a lot of that when looking at paging/infinite-scroll since that will inform me of the best way to store it.

I added some FIXMEs for things that should be fixed, but are fine to handle in a different PR.


The splitting of editor views is clashing with this code. I'll have to look at it this morning and figure out what is incompatible - I think there is a minor change I can roll back and reapply to sort it out.