Fixed source_f crash when looking for untranslated words (remaining source_f cleanup). Ref #INTL-1567

The work done in https://github.com/Yelp/pootle/pull/3 excluded the source_f attribute from both UnitForm model and edit unit template. This changed though fixed the source_f corruption issue, it also introduced a bug that caused the /translate.html view to crash resulting in an infinite loader.

Things I did here:
- Reintroduced https://github.com/Yelp/pootle/pull/3 (which was reverted when when "remaining words" bug was found)
- Search the entire source code for uses of source_f attribute. This time with special emphasis on non python code e.g. javascript modules, which is something we missed in https://github.com/Yelp/pootle/pull/3
- Change the editor.js module to verify the html element `id_source_f` exists before attempting to access its attributes.
- Augmented the interactive testing and manual verification to cover those use cases we missed, since sadly, we don't have a working test suite for version 2.5.0.
Same question here.