New FormLabelSuffixMixin to override the annoying Django default ':'

I made this long-overdue mixin to get rid of the awful trailing ':' in our field labels. By inheriting the mixin on a form class, you can be assured that the label_suffix for all fields within that form will default to an empty string. The mixin also allows you to declare the label_suffix string for all fields at a form class level, as a simple attribute.

Note that custom calls of label_tag on a BoundField are not affected; the label_suffix specified in such an invocation will be respected.

Other changes include a settings_no_coverage file for streamlined testing without coverage, and a switch to unitest.TestCase for code that doesn't touch the database.