Restrict linkify shorthand expansion to plain text

[hlfan]

Define linkify link detection/expansion rules only in the Text submodule and add a test against it messing up markdown content, which

• fixes Linkify breaks Overpass QL source code blocks #6643,
• fixes Diary comments expanded to include text I did not write #6673, and
• fixes Undocumented changing of message content #6692.

[pablobm]

I wonder if the Markdown versions should still linkify, just not within code blocks. More difficult to implement though, so not for this PR.

[hlfan]

You know what? I'm splitting this PR. I originally just wanted to get some ideas down while in that headspace, but this is holding up the bug fix.

[tomhughes]

The new cut down version of this looks good to me, thanks.