Implement ID forwarding (like email .forward files) (SPEC-156) #65
Description
We want people to run their own homeservers.
If people get too attached to their @foo:matrix.org IDs, they may never want to run their own homeserver.
So we need to specify and implement the ability for @foo:matrix.org to point through to @foo:foo.net so that the user can actually exist on their own HS.
(Imported from https://matrix.org/jira/browse/SPEC-156)
(Reported by @ara4n)