Minor Django 2 fixes; remove unused SiteMember model

• Remove unused SiteMemberBackend and its model
• add app_label to models: fix breaking tests
• minor repo updates that doesn't affect the package itself when used in production/testing

Juniper compatibility: Support Django's AllowAllUsersModelBackend (fixed git tag)

This release fixes an issue with inactive users not getting a clear error message RED-1864.

Related PR: #13.

Note: Please use this release instead of 5.2.0-appsembler12 because the latter has the incorrect Python version set in the setup.py file which will show an incorrect version in pip freeze.

Juniper Tahoe edx-organizations

See https://appsembler.atlassian.net/browse/RED-1491

Fixes for admin performance

Merge pull request #4 from appsembler/omar/admin-perf-fixes

Admin performance fixes for UserOrganizationMapping

Fixes for Hawthorn and Django 1.11 -- with version bumped

0.4.12-appsembler4

Bump the version to 0.4.12-appsembler4

Fixes for Hawthorn and Django 1.11

Releasing #3

Fixes for Hawthorn and Django 1.11 -- with version bumped

0.4.12-appsembler2 didn't have correct version bumped in the code. This fixes that.