Allow users to be blocked from posting jobs [PD-605]

There is no UI for revoking a block.

All tests pass.
Null only refers to how it's stored in the database. Since ManyToManyFields require a table to establish the relationships between two other tables, the way you would specify that there is no relationship is by not adding a row in the intermediary table.

I still could be wrong about what Django is trying to do to it, but from a db perspective it's meaningless.