Renamed selector for clarity.
Reordered managers/methods to align with the style guidelines.
Added get_absolute_url for units.

The URL points to the unit in the translation interface.
Use arrays instead of string concatenation.
Provide more data to the editor, store pootle_path once.
Pass fields as a comma-separated list.
Implemented export view for proofreading by translators.

Fixes bug 2845.