Use Django database router API

[Archmonger]

By submitting this pull request you agree that all contributions to this project are made under the MIT license.

Description

By default, all databases registered within Django are configured with identical columns. There may be some edge cases (#152) where this can be problematic. In order to to stop ComponentSession from existing on databases other than the user configured REACTPY_DATABASE, we need to create a Database Router.

Changelog

• reactpy_django.database.Router must now be registered in settings.py:DATABASE_ROUTERS whenever settings.py:REACTPY_DATABASE is set.
• Added system checks for a variety of common ReactPy misconfiguration

Checklist:

Please update this checklist as you complete each item:

• Tests have been included for all bug fixes or added functionality.
• The changelog has been updated with any significant changes, if necessary.
• GitHub Issues which may be closed by this PR have been linked.

[rmorshea]

@numpde would you be willing to help review this? I'm just not familiar enough with Django to understand the details here. I can cover a high-level review though.

[Archmonger]

I'd like to get these system checks out ASAP, so I will merge this. If you decide to provide a post-merge review I will incorporate any changes in a follow up PR.