Be able to load URL in non-TEMP mode

[david-a-wheeler]

When loading a URL, it'd be nice to not be forced into TEMP mode.

One approach:

If loading from a URL, and there's already some content, ask the user "Do you want to load this into the main work area or TEMP mode? TEMP mode is a separate work area intended for review other work; edits in TEMP mode are thrown away when you leave its tab."

That would let people share a URL and then continue where that left off. It also lets me do my "reset trick" where I load an empty state to reset things.

It'd also be nice to be able move "current TEMP mode contents" into the main work area.

[david-a-wheeler]

Maybe say "regular mode or TEMP mode" to be consistent with other wording.