17929 Add Scope Mixins to Prefix

[arthanson]

Fixes: #17929

Had to move the filterset mixin to it's own file as there was a circular dependency - this requires updating previous imports that reference it.

Note: The mixin uses %(class)s for the related_name which resolves to "_prefixs" instead of "_prefixes" which isn't great, but it is an internal field that shouldn't really be references, so shouldn't be much of an issue.

[jeremystretch]

Note: The mixin uses %(class)s for the related_name which resolves to "_prefixs" instead of "_prefixes" which isn't great, but it is an internal field that shouldn't really be references, so shouldn't be much of an issue.

IMO this is going to cause problems in the future, as we're forcing ourselves to accommodate what is essentially a typo. I propose adopting one of three options:

1. Use Django's default related name convention (prefix_set).
2. Drop the related name entirely for these fields.
3. Override these fields for the Prefix model to declare related_name explicitly.