Remove Flask Assets

This commit’s parent introduces a Node-based way of managing front end assets.

Nothing else is needed in the app, specifically not Flask Assets. Flask itself
automatically routes requests for `domain.tld/static/*` to the files in
`app/static`.

This also removes all the Ruby stuff.

app.py

@@ -1,8 +1,6 @@
 import os
-from flask.ext import assets
 from flask.ext.script import Manager, Server
 from flask_migrate import Migrate, MigrateCommand
-from webassets.filter import get_filter
 from app import create_app, db