Don't keep filters when clicking on context rows.

There could be no matches for units from context rows while keeping the
filtering criterion, leading to queries with no results and unexpected
behavior.
Plain ol' js just works fine (and it's more performant).
Remove title from navigation buttons in case they are disabled.

Avoids inconsistent tooltips.
Filter submissions made from the web interface.

Unless this filtering criterion is applied, there were occasions were
languages and translation projects were displayed with all the numbers in
the row being zero.
Don't display zero in contribution tables.

Also simplified the returned data structure.
Filter on PootleProfile objects, not on User objects.

Surprisingly this was previously working but only for some user profiles.
Require Translate Toolkit >= 1.10.0
Add notes about bugzilla
Fix URL
Selling mistakes
PEP8: cleanup
My simple pep8 cleaner
PEP8: W293 blank line contains whitespace
PEP8: W602 deprecated form of raising exception
PEP8:  E301 expected 1 blank line, found 0

Improve readability
Add help text
PEP8: E225 missing whitespace around operator
PEP8: E227 missing whitespace around bitwise or shift operator
PEP8: E231 missing whitespace after ','
PEP8: E271 multiple spaces after keyword