* remove the /api/database/validate endpoint
* introduce connection checking as part of the database api create/update endpoints directly
* modify the return of the Database model in the api so that if a password was provided we swap it out for a known value and prevent leaking of the real password via the api.

migrate existing installations and automatically set the `:admin-email` if it hasn't been set before.

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.