Update the account data event m.direct after accepting an invite

[alfogrillo]

The matrix protocol specifies that clients (both the creator of a DM room and joiners) are in charge to flag the room as DM chat.
This is being done in two places but using the same mechanism: account_data (event m.direct).
On rooms' creation we already update the account data automatically (done in Rust).

This issue is about updating m.direct when accepting an invitation for a DM chat.

Behaviour
When accepting an invitation the user looks inside his invite (an event of type m.room.member) and check under the path content.is_direct if the flag is set to true. If yes, he has to register the room id in his own account_data inside m.direct just after he joins the room.

[jplatte]

This should probably be done here.