Add ``HubRequest`` model for apply-to-be-a-community functionality.

https://github.com/madewithbytes/us_ignite/issues/94

The model will be used to track the application process.
Swap ``Hub`` for ``HubRequest`` in the application form.

https://github.com/madewithbytes/us_ignite/issues/94

The new model has exactly the same required fields for
the application.
Add mechanics to submit an Ignite community request.

https://github.com/madewithbytes/us_ignite/issues/94

Any registered user will be able to apply to become
a Ignite community by filling out the application form:
 - Name
 - Website
 - Summary
 - Description

The feedback of the application will be done in the
``notes`` field.