[WIP/Proof] Filter Labels

[saxamaphone69]

Another one that is incomplete, but perhaps is a good starting point for others to pick up.

Best to do this on a fresh install and use some filters like /what/ or /something/ that don't always hit, within a thread.

Notable issues at this stage include:

• I don't actually know what I'm doing, so not sure if I'm referencing things correctly
• Within the Label menu, entries are not given the .entry class like other submenu items
• Sometimes a random true is added to the Label list, not sure where or how
• Threads probably don't work
• Unhiding a post doesn't update the label list correctly
• As mentioned in Filter hiding cancels out highlighting #3359, posts currently can't be marked hidden AND highlighted
• Haven't tested multiple highlights
• Perhaps clicking on the matching filter label should take you to that line in the filter list?

Perhaps not in the scope of this PR, but I suppose there should be a setting that shows the labels in Stubs, some form of overriding, and being able to detect if a filter is hiding all posts (warning on initial incase it was an error, allow it to be dismissed if someone really never wanted to see posts that use the letter "a").