Don't lose geometry edits when changing modes

[dabreegster]

See #247 for context. This PR does a few things:

• If you're drawing something new and switch to another mode (by clicking something in the sidebar, or clicking another draw tool), the new object is added anyway, but the form isn't opened. The user doesn't unexpectedly lose work.
• If you're in edit geometry mode and switch to another mode, the unsaved changes do get saved.
• The existing "Cancel" buttons continue to work as expected. For new objects, they totally throw away whatever's drawn. For editing, they revert to the previous state of the object. This lets the user undo any accidental changes.

These changes are for polygons and linestrings, but not points. The point tool is trivial; the user just has to click the map to place or move something. If they go click another button somewhere, there's no reasonable "last edited state" to save.

Manual testing seems rock solid, but I'm working on new Playwright tests now.

[dabreegster]

Adding Robin as a reviewer for UX / "is this nicer?" check: https://acteng.github.io/atip/autosave/scheme.html?authority=Adur

[robinlovelace-ate]

Adding Robin as a reviewer for UX / "is this nicer?" check: https://acteng.github.io/atip/autosave/scheme.html?authority=Adur

Will try now...

[robinlovelace-ate]

Tested and seems to work great to me. Great work and many thanks for detailed comments on the code, will help onboard others to the codebase and make code review easier (although I'm not in a position to review the code)!