Showing Mutual Rooms

[ShadowJonathan]

This is a request to implement MSC2666 across Element.

Previously, matrix-org/matrix-react-sdk#4897 had been in the works to add this to element-web, but it did not reach completion before I picked up MSC2666. Afterwards I found it hard to resume that PR.

This feature has already been added to js-sdk (matrix-org/matrix-js-sdk#1417 and subsequent iterations).

This issue is a request to implement it in respective SDKs across Element, and to come up with a common design for this, likely stemming from the work in matrix-org/matrix-react-sdk#4897.

[Gnuxie]

Dying for this feature, how hard can it be to copy the RoomFacePile that is used for spaces to show common rooms? Maybe easier than scavenging the preview part from matrix-org/matrix-react-sdk#4897?