Keyword assignment processing test.
Dropped a piece of keywords code that doesn't seem to be needed any longer.

Moreover, the Manager.fallbacks() never have even become a pull request
(supporting query fallbacks is feasible, outdated branch with a sketch can
be found on github: feature/filtering-with-fallbacks).
All Mezzanine URLs should use language prefixes.
Revert "All Mezzanine URLs should use language prefixes."

This reverts commit 235d5cb638b29f1410c15d1ae4a6ccfe14aff6e0.
First documentation sketch.
Fixed a type in an inline comment in generic/fields.py.
Factored out deleting unused keywords as a manager method.