Django-ckeditor-link

- adapted tox and github actions for django 3.2
- removed django 1.11 from tests (will probably still work, but not supported officialy anymore)
- small bug with POST variable encoding solved: verify url was called with &var[]1&var[]=2 when there was an array.
this was solved using $.params (now using &var=1&var=2, what works with django).

- more tests added, mostly templatetag/link model stuff
- re-introduced django 1.11/django-cms 3.4 compat, for legacy projects

- drop python 2.7 support
- prepare django 3.0 support (wait for django cms, try at your own risk)
- multi site django-cms bug fixed, with test coverage

- optimizing padding in ckeditor link dialog and django 2 and up

- fixed a nasty get_link_target bug, for the provided base link model
- tox / travis refactoring

- drop support for django 1.9 and below
- builtins fix for python2 / cmsfilerlinkmodel
- some link model tests, and fallback enhancements for page/cms_page change