Display a summary of shared rooms in the invite dialog

Because I don't know everyone's MXID by heart, each time I'm inviting someone to a room I worry that I'm going to invite some random people that happens to share a common display name with the user I want to invite, and sometimes have to switch back end forth between the dialog and another room just to look at the MXID of that user.

I think a nice way to improve this would be to display a summary of the rooms shared with each user that's suggested in the dialog. This could be done using [MSC2666](https://github.com/matrix-org/matrix-doc/pull/2666), which is already implemented in Synapse, and in the matrix-js-sdk.