Tidy up the commit structure for browser updates UI

This commit is slightly outdated: Bug 16940: After update, load local change notes.

Commit message

Also fix bug 29440. Now about:tbupdate is styled as a fairly simple changelog page that is designed to be displayed via a link that is on about:tor.

We no longer link to "about:tbupdate" anywhere.

Maybe the comments about the content/chrome separations are a bit unnecessary, since that pattern is standard now and followed in other places.

`BrowserContentHandler.sys.mjs`

The changes to BrowserContentHandler.sys.mjs are the only part of the commit not related to "about:tbupdate". It should probably be moved to the "about:tor" commit that actually uses the torbrowser.post_update.shouldNotify preference: Bug 7494: Create local home page for TBB.

This comment about the use in "about:tor" is outdated.

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