[WIP] Inline widgets

[turt2live]

Review with https://github.com/matrix-org/matrix-react-sdk/pull/3252

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions[bot]]

This PR has been automatically closed because it has been stale for 180 days. If you wish to continue working on this PR, please ping a maintainer to reopen it.