Add support for reseting local database.

Since the models are not finalized is useful to
reset the database often instead of generating
several migrations.
Add unique slug field for the profile.

https://github.com/madewithbytes/us_ignite/issues/45

Each profile should have a URL, the short UUID
field will generate an short unique autoslug for
the user.

This can be amended in the future and be a chosen
slug.

Refers #45.
Added documentation for reseting the database.
Add model admin to the profiles.

The configuration of the admin helps to diagnose
the sign up and user updates.
Bugfix: Make sure the slug is not recreated on every save.

Without validation for the existing value the field
would be recreated on every save. Now once the slug exist
the field won't try to generate a new one.