Room directory should default to searching user's HS if unconfigured

[jryans]

There's a section in config.json:

    "roomDirectory": {
        "servers": [
            "matrix.org"
        ]
    },

This dictates which homeservers appear in the dropdown in the room directory.

We should probably default to searching the user's own homeserver if nothing is provided in that section of the config.

Originally posted by @lampholder in #9166 (comment)

[jryans]

@YaoiFangirl420 Does your fix from matrix-org/matrix-react-sdk#2821 resolve this issue? It looks like it might, since we already add the user's HS to the server list, and we might have just gotten blocked by the bug you fixed there...

[jryans]

I am fairly sure this works after matrix-org/matrix-react-sdk#2821, so I'll close this.