provide information about our `:engines` through our main MetabaseSettings which simplifies some things and removes the need for a /api/database/form_input endpoint.

Fixes #1421.

make it possible to complete the initialization process from outside the metabase.core namespace and trigger it in our test setup process so that logins work for unit tests again.

add a quick conditional to our current-user-id middleware to ensure it doesn't attempt to validate the session if we haven't initialized the app yet.