This is a second attempt at: #33001

We had to revert the commit because it was performing a site-setting check at boot time, which is prone to issues and not allowed.

This PR:

• re-introduces the changes in the original PR
• a fix by not performing a site-setting check at boot time (verified by: SKIP_DB_AND_REDIS=1 DISCOURSE_DEV_DB="nonexist" bin/rails runner "puts 'booted'" locally and should be caught by the new CI check introduced here: DEV: Add SKIP_DB_AND_REDIS bootability check to CI #33158)
• adds a fix to the translation editor to not show the original post locale in the dropdown, as well as adding an indicator of what the original post locale is in a small badge in the header:
  •