Meta-issue: Show list of shared rooms with another user

[Half-Shot]

This issue will be used to track the various PRs I've written to add a shared room list into the user panel.

matrix-react-sdk PR: matrix-org/matrix-react-sdk#4897
matrix-js-sdk PR: matrix-org/matrix-js-sdk#1417
synapse PR: matrix-org/synapse#7785
MSC: matrix-org/matrix-spec-proposals#2666

keywords: mutual

[Mikaela]

Keywords: common groups (and common rooms or rooms in common), shared groups. I found this with my third search attempt.

[ShadowJonathan]

Additional keywords: "Mutual servers" from discord

[ShadowJonathan]

This came up in HQ: it'd be nice if users could also be able to "filter" DMs to ones from people they have a room in common with, and "delegate" the rest to a "bin" which is only accessable via a menu, sorta like Spam