Add filtering by tags to forums lists

This adds thread filtering functionality to forum list views. Functionally it's fine (with the below caveat), but maybe TheDave or Diabolik might want to knock up a quick "filter" icon rather than using the built-in search icon, and also pick a better colour than yellowColor for the "active" state.

There is one outstanding issue; if you filter a forum, kill the app and come back within the 15 minute refresh timeout period, the filter will still be active but the icon isn't coloured to show that a filter's active. I think postIcon.composeID should be persisted to fix this (and checked when the view initialises), but I've never used CoreData and have heard scary things about modifying the data models, so I think it's safer for me to leave it to someone who knows what they're doing. Alternatively if there are ideas for an alternative solution I can take a look.