Django-xmpp-http-upload

* Add support Django 2.2 and Django 3.0.
* Add support Python 3.7 and Python 3.8.
* Add support for djangorestframework 3.9, 3.10 and 3.11.
* Drop support for Django 2.0 and Django 2.1.
* Drop support for Python 2.7 and Python 3.4.
* Drop support for djangorestframework 3.7.
* Return `Content-Type` and `Content-Length` headers with direct download.
* Ensure test coverage of 100%.

* Increase length of `type` column to 255 characters (we sometimes encounter up to 80 characters).
* If a user does not match any ACL from the `XMPP_HTTP_UPLOAD_ACCESS` setting, all uploads are denied.
* Set the default app name to ``xmpp_http_upload``.
* Add many more tests to the test suite.
* Use isort to sort imports, increase textwidth to 110 chars.
* Add new setup.py commands to run the test-suite or code quality tests.
* Add new setup.py command to generate a coverage report for the test suite.
* Use [Travis CI](https://travis-ci.org/) to run the test-suite and code quality tests in all supported Python
and Django Versions.