Add test coverage to confirm that install buttons are disabled or invisible while an installation is in progress

Tagging as a beta blocker because this adds very necessary test coverage that our existing UI works as intended.

Assigning to @chrisfromredfin for some manual testing. You should see basically no behavior change here (it's all in the installation-related frontend code).

both places. while this does let you page away and back while losing status, the larger dragon will handle that