fix: move model base class out of model subpkg

and other things, used by other packages (rattail)

otherwise when rattail imports them, the whole WJ model comes along
with it and it can interfere with sqlalchemy-continuum versioning

docs/api/wuttjamaican/db.util.rst

@@ -0,0 +1,6 @@
+
+``wuttjamaican.db.util``
+========================
+
+.. automodule:: wuttjamaican.db.util
+   :members:

docs/api/wuttjamaican/index.rst

@@ -17,6 +17,7 @@
    db.model.base
    db.model.upgrades
    db.sess
+   db.util
    email
    email.handler
    email.message