Implement user_permissions model, api, and test

[ethanstrominger]

Fixes #22

What changes did you make?

• standard changes:
  • models.py
  • serializers.py
  • views.py
  • urls.py
  • admin.py
  • 00025_user_permissions_and_more.py
• constants.py: new file for holding constants for code for practice area admin and project lead. This will be utilized more going forward.
• 0004_perimission_types_seed.py: modified to populate rank. Any databases that applied this will need to reverse it. I will need this field in another pull request, better to populate it now.
• conftest.py. Added fixtures: user_superuser_admin, user_permissions (2 generic user permissions), user_permissions_admin_project, user_permissions_practice_lead_project
• test_api.py
  • fixed test_create_permission_type which would otherwise create a duplicate record
  • added test_get_user_permissions. Corrected sincelast review to get the count from UserPermissions for better stability.
  • added test_user_permission_admin_project and test_user_permission_practice_lead_project to check the str functions under different scenarios