Update User model and add vendor/realtor specializations

[Justin-Ferwerda]

This PR extends the user model and adds data for vendor and realtor specializations re: #27

1. I also created a new model, UserLicense, as a user can be licensed in multiple states. After some research, it seems as a realtor can be licensed in one state, and have "reciprocity" in other states, so they likely share a license number. I put the license number on the UserLicense model just in case if you are licensed in multiple states you have different license numbers for each state.
2. I added state info in fixtures for all states and added abbreviations in case that would be helpful for FE.

to test:

• I deleted my migrations and my db, as extending the user model was a breaking change.
• You will need to have 3 auth-user instances in your database with pks of 1-3.
• git fetch origin update-user-model
• git checkout update-user-model
• python manage.py makemigrations
• python manage.py migrate
• python manage.py loaddata content_types states tags counties specializations user_types users user_specializations user_counties content_tags contents user_licenses

These changes shouldn't cause anything to break on FE, you just have more user data to display at this point.