Release 3.0.0b0

This release removes some old things we don't use anymore and ensures compatibility with Django 3.1 and python 3.9.

Breaking:

• Remove webpack_constants management command and related code
• Remove flatten_urls and related code
• Remove language file generation and related code

Also:

• Replace internal usages of url with re_path
• Replace internal usages of ugettext_lazy with gettext_lazy
• Add tests for python 3.9 and django 3.1
• Use Github actions for CI instead of Travis
• Use poetry for releases and dependency management
• Replace flake with prospector
• Add black for automated code formatting
• Add some tests for account apis