Component Initial HTML

[Archmonger]

The idea of this PR is to allow for customizable initial HTML.

This is useful for...

• Showing a simple loader/message in situations where a websocket connection could not be formed (Message display can be delayed via CSS).
• Can be used to run simple scripts on the component div.

This is different than an IDOM Core idom.component fallback=... that we have discussed due to the fact that this implementation fallback has no WS reliance.

[Archmonger]

I want to mention that this PR is currently nonfunctional due to mountViewToElement wiping the inner contents of the component div on WS connect. In order for this PR to work, this behavior in IDOM Core would need to be changed to clear the HTML on first render instead.

Mostly opening this PR as a discussion as whether or not this concept can/should be pursued.

There are several situations where it'd be useful to have a fallback that is not reliant on websockets.

[rmorshea]

I think this can be addressed once reactive-python/reactpy#581 is completed.

[Archmonger]

I'm going to close this in prioritization of SSR within IDOM v3

• SEO Compatible Rendering (Pre-rendering) reactpy#578