test: fix broken test

now that we are providing a default value for mail template paths. i
could not think of any reason we were *not* doing that before...?

Fix broken test

not that it tests much anyway

Move tests out of the installed package set

this seems to be better practice. from what i can tell this is a
clean move with no breakage.. and built package files no longer seem
to include test modules

Overhaul email framework.

At least, mostly..still have a gap here and there.

Overhaul config system, finally replacing edbob (yay!).

* No more `edbob.init()`, now we use `rattail.config.make_config()` instead.
* Adds new `ConfigExtension` mechanism to replace old 'init' config magic.
* Adds config extension for `rattail.db`, to load all Rattail db engines.
* All application entry points have been updated to use `make_config()`.

This also refactors `rattail.db` somewhat in general, to allow for safely
importing it without getting SA-related import errors.

Add config setting to globally disable sending of emails.

With tests! Had to refactor the mail logic a bit to make that part easier.

Let mail templates be configured as "resource paths".

I.e., let paths be specified as relative to a Python package.