Better view filter management

[KodrAus]

Some installs need to apply view filters to lots of users. This can be done through Seq's API, but is clumsy through the UI itself. View filters also aren't indexed so users who can't see very recent data will end up with slower queries.

Signals seem like a perfect fit for sharing view filters, but since signals are visible to users it means a user could technically inspect (and possibly edit) their own view filter that way. We could consider a new entity view filter entity type that's like signals, but outside of their sharing model. These view filter entities would participate in indexing just like signals.