Formalise rules for picking existing 1:1 conversations (SPEC-413)

Vector has a "start chat" button which is supposed (modulo https://github.com/vector-im/vector-web/issues/1613 ) to re-use existing chats, and in general it should be possible to send a message to someone without first going through a room-picking dance.

This could take the form of a list of rules for clients to follow, or an API which picks a room for you.

(Imported from https://matrix.org/jira/browse/SPEC-413)

(Reported by @richvdh)

Start by reviewing the Vector issue 1613 and the existing discussion around reusing 1:1 conversations. Define whether the specification should provide client rules or an API for selecting a room, and document the expected behavior for sending a message without room selection. Done means the approach is formalized clearly enough for clients to follow.

Written by the indexing model from the issue text.