AG housekeeping (#49047)

* upgrades is-airgapped? defn to defsetting airgap-enabled

- uses the airgap-enabled settings value to remove Cloud section in
  SettingsEditor
- "Metabase Enterprise Airgap" -> "Enterprise Airgap"

* hide still need help section in airgap mode

* move flag checking into selectors

* fe lint

* ts type check

* use optional chaining and nullish coalescing

* hide StillNeedHelp in ag mode

* adding airgap-enabled to createMockSettings

---------

Co-authored-by: Nick Fitzpatrick <nickfitz.582@gmail.com>