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.
This code is only used by the "Translation Memory" subsystem (suggest translations). This are automatically retrieve when the /translate.html view is displayed.

We could potential remove the call to this code, so it wouldn't fetch the suggested translations. It is a plausible approach. 

I was aiming for a more backwards compatible approach, something in which we didn't have to kill the entire feature. This way, in case it's being used somewhere else, it would still work as expected. Probably a bit ambitious, but I think it is less intrusive. In my opinion, regardless of this bug, the code should have the check for 'undefined' before trying to access the object's attribute.