GitLab

- PFPModelForm now merges attribute/parameter disabled fields with
  those generated from a user parameter.

- Added tests against Django 2.x, and it's supported Python versions.
- Added tox test matrices and updated gitlab-ci to use these.
- Moved some cache manipulation functions into utils module

Interpretation of doco that anon users have is_active == False is
incorrect, handle anon users properly.

Change license to Apache-2.0, update README.
Revert change to merging permissions, return to original overwrite
approach.