Various bug fixes found while testing SaaS build (#6459)

# Description of Changes
Various fixes and improvements I made while testing the SaaS code:
- Changes the new `.env.saas` file to live in `app/` and match the
semantics of the other `.env` files
- Adds top-level `task dev:saas` command to spawn SaaS frontend &
backend
- Deletes dead SaaS code and improves some overriding logic
- Fixes refreshing issue when coming back to the tab
- Fix the Compare tool's selection logic
- Make Compare handle error cases properly
- Fixes the location of the "Dismiss All Errors" button (was rendering
on top of the top-bar with a transparent background previously so it
looked rubbish)
- Fixes file selection in PDF Editor