MSC4095: Bundled URL previews

[tulir]

Rendered

Implementations:

• Beeper Desktop, Android and iOS render incoming bundled preview data, and bundle previews in outgoing messages using data from the /preview_url endpoint.
• The next-gen Beeper Android client also generates preview data locally instead of asking the homeserver to do it.
• mautrix-{whatsapp,signal,imessage} and beeper-imessage bridge preview data in both directions.
• mautrix-{discord,telegram,twitter,instagram} bridge preview data from the remote network to Matrix.

Signed-off-by: Tulir Asokan tulir@maunium.net