Stabilise MSC4278 Media preview controls

https://github.com/matrix-org/matrix-spec-proposals/pull/4278 is now considered stable. Given this involves cross-device account data we should consider *both* configs (with a preference for the stable config) for a transition period until such a time the old config can be removed.

Start by reading MSC4278 and search the Element Web code for the Media preview controls and the cross-device account-data configuration. Trace how the stable and old configurations are read during the transition period. Done means the stable configuration is preferred while both configurations remain supported until the old one can be removed.

Written by the indexing model from the issue text.