only cache units and courses within running web application

[davidsbailey]

the impetus for this change is that I determined that #71016 broke during seed on test and production because Unit.should_cache? is true when seeding in those environments. There's no reason for us to be caching at that time, so the solution is to disable caching when seeding.

Then I saw some issues with the logic in Unit.should_cache? and wanted to clean it up a bit further.

Done in this PR:

• stop caching during seeding or any other rake tasks
• start caching within the running web application in drone ui tests (this could speed up drone runs)
• simplify by further limiting caching to only happen within the running web application
• remove obsolete SKIP_SCRIPT_PRELOAD and bin/dashboard-console-skip-preload

Caveat: one possible controversial issue here is that we will no longer be caching within rails console. I've solicited input on whether this is an issue for anyone in slack.

Testing story

• passing drone run gives good coverage for CI, in particular the ui test steps which now have caching enabled
• problems in staging, development and levelbuilder are unlikely because caching was already disabled there
• problems in unit tests are unlikely (in development or test) because caching was already disabled for unit tests

Deployment notes

• after merging, I will notify the DOTD and keep a close eye on the next DTT and DTP (ci_build cronjob, deployment rake tasks, other cronjobs)