First off: This commit changes the default development database
backend to mysql. sqlite, however, is still completely supported
with the caveat that a user must now modify config.dev to use
the sqlite backend.

While looking into this, it was discovered that our SQLAlchemy
backend for mysql (mysql-connector) completely broke model
attributes when we switched to utf8mb4_bin (binary) -- it does
not correct the correct conversion to and from binary utf8mb4.

The new, replacement dependency mysqlclient does. mysqlclient
is also recommended in SQLAlchemy documentation as the "best"
one available.

The mysqlclient backend uses a different exception flow then
sqlite, and so tests expecting IntegrityError has to be modified
to expect OperationalError from sqlalchemy.exc.

So, for each model that we define, check keys that can't be
NULL and raise sqlalchemy.exc.IntegrityError if we have to.
This way we keep our exceptions uniform.

Signed-off-by: Kevin Morris <kevr@0cost.org>