Implement remove buttons on own post tags

[marcustyphoon]

Description

Core implementation of a feature I've wanted for a long time, but that is somewhat niche: the ability to remove tags from a post with one click. This is very nice when tags are automatically added to posts based on their source (i.e. #Youtube) but is otherwise not useful very often.

Resolves #549 (which I intentionally let go stale).

Things one should debate:

• If merging this functionality, should it be part of Quick Tags, be its own feature, be... a tweak (which probably makes the most sense scope-wise but completely violates the norms for how complex a tweak is), etc?
• Should every tag on an editable post be removable or just the auto-added ones? What is the list of auto-added ones?

Ultimately my guess is that no one can come up with a justification for implementing this, but hey, I made it.

Testing steps

[marcustyphoon]

• [ ] #1980