REST: Rename Filter -> FilterSet

REST: Rename Filter -> FilterSet

FilterSets are to Forms as Filters are to Fields: the former is
made up of the latter. We have a FilterSet for each resource that we
wish to support filtering on (i.e. all of them).

Rename our "Filters" to the more appropriate FilterSets.
The old name was confusing and will conflict with some forthcoming changes.

Signed-off-by: Stephen Finucane <stephen@that.guru>
[dja: commit message]
Signed-off-by: Daniel Axtens <dja@axtens.net>