On-update behavior: Remove all extension DOM elements

[marcustyphoon]

Description

see #1550

Testing steps

[marcustyphoon]

On reflection, places where this would be a problem unless we specifically declined to use dom() or added an argument to it or alternative util export whose result would not be removed on extension reboot:

• Subtle Activity Mutual tweak, where we empty the contents of the real text node
• AccessKit Disable GIFs, where we wrap tumblr elements
• Collapsed Queue, where we wrap tumblr elements
• Maybe Classic Search? I don't remember.

This would also do inconsistent things with, for example, Shorten Posts, which needs a class removal to undo part of its effect.

Going back to #451 (comment) as the best-so-far way to handle this.