Fix page title not being updated when navigating

[marcustyphoon]

I thought I remember instances in which the React code modified the title element rather than replacing it (and why wouldn't it, after all, if that's the minimal DOM diff). That being said...

Originally posted by @marcustyphoon in #1360 (comment)

Yeah, I don't know why I ever believed we wouldn't have to handle that case.

Description

When navigating from one tagged page to another, with our "Hide the Home/Following unread count" tweak enabled, the page title is not updated due to limitations in the code that detects the page title changing.

This fixes this by observing changes to the title element as well as its replacement, as my original code did.

Resolves #1613.

Testing steps

• Enable the "Hide the Home/Following unread count" tweak
• Navigate to a tagged page
• Navigate to another tagged page by clicking on a tag or clicking a Tag Tracking+ sidebar item
• Confirm that the page title corresponds with the current page (and does not include an unread count)