condense the `(process-*)` functions in the qp namespace into a single function definition and unify the call stack so that we simply apply all of our qp middleware functions regardless of the query.  this simplifies the understanding of the qp code path and reduces a bit of redundant code.

the one requirement that comes out of this is that our qp middleware now needs to add some conditional code so that it only applies itself on queries that it's meant to work against.

update our predicate function `field-should-have-field-values?` to include a check that the field is not one of our Date/DateTime/Time field types.

* 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.