m.room.aliases may contain aliases from other homeservers

[richvdh]

There is no validation on the aliases listed on an m.room.aliases event, so it is possible to list things which aren't valid aliases at all, or which would be valid aliases, but are on a different server:

{
  "type": "m.room.aliases",
  "state_key": "matrix.org",
  "content": {
    "aliases": [
      "random jakj # 2342 !",
      "#foo:sw1v.org"
    ]
  }

Both of these currently confuse riot-web at least.

Related: #176, #533