Backport `reactpy_django` features to `reactpy`

[Archmonger]

Current Situation

Currently, ReactPy-Django is significantly ahead of ReactPy in terms of feature set.

While some of ReactPy-Django's features can't exist on ReactPy (mostly due to the lack of a built-in database), a few of them can.

Proposed Actions

The following feature can be backported:

• Client Side Python Components #110
• Improved default transforms for vdom_to_html #1278
• Convert vdom_to_html to reactpy_to_html
• SEO Compatible Rendering (Pre-rendering) #578
• Support offline HTML fallback within template tags
• use_query and use_mutation hooks (NOTE: This should probably become an external package)