Effectively fixes #11074
Effectively fixes #7112
Fixes #6930
Fixes Jitsi widgets not working for guests (#8933)
Fixes #5048

Previously we were relying on an integration manager to be defined, functional, and alive in order to join Jitsi calls. This commit changes this so we aren't reliant on an integration manager for Jitsi calls at all, and gives people the option of choosing a Jitsi server via the config.json.

This side is just the wrapper/shell: the logic is mostly in the react-sdk (to be linked via PRs). This layer simply has an HTML file exported that can be used to render a Jitsi widget, and the react-sdk constructs a URL to access it locally. This is similar to how the mobile apps handle Jitsi widgets: instead of iframing the widget URL directly into the app, they pull apart the widget information and natively render it. We're effectively doing the same here by parsing the widget options and using our local wrapper instead of whatever happens to be defined in the widget state event.

Integration managers should still continue to offer a widget URL for Jitsi widgets as this is what the spec requires.

A large part of this is based upon Dimension's handling of Jitsi and widgets in general: a license has been granted to allow Riot (and therefore the react-sdk) to use the code and be inspired by it.