Allow use of Django 3.0

[delsim]

Remove constraint requiring use of Django 3.0
Update the documentation for X_FRAME_OPTIONS to permit use of iframes

[ghost]

I have a project requiring Django 2.1. Installation process for django_plotly_dash >=1.3 seems to install Django 3.0 as well. Moving forward, is this project dropping support for Django 2.x? The documentation still seems to suggest that 2.x is supported. Is there a reason the requirements.txt file can't be changed to Django > 2 (or whichever 2.x versions are explicitly supported)?

[GibbsConsulting]

@sleep-rpm there is #246 which will address this issue - pull requests most welcome - and also #215 talks a bit about a workaround, which is to use version 1.2.0 of django-plotly-dash. This is for Django v2.2. You can always force versions of packages in pip, but the dependencies will get messy quite quickly!

The Django project will support v2.2 until April 2022 as it is a long-term support release, but v2.1 is already past the end of its extended support. We won't be explicitly supporting a release series that has already been EOL'ed.

Commercial (ie paid) support is possible; this is available from us as well as others.