Resolve conflict where Matrix room exists with alias but we don't have it stored yet

Resolve conflict where Matrix room exists with alias but we don't have it stored yet (consistency problem). This solves another class of problems we are seeing with the Gitter -> Matrix import process:

Error occurred while backfilling events for opts.uri=undefined gitterRoomId=xxx: matrix-appservice-bridge/matrix-bot-sdk threw an error that looked more like a request object, see {"statusCode":400,"body":{"errcode":"M_ROOM_IN_USE","error":"Room alias already taken"},"headers":{"date":"Fri, 20 Jan 2023 07:55:14 GMT","content-type":"application/json","transfer-encoding":"chunked","connection":"close","cache-control":"no-cache, no-store, must-revalidate","access-control-allow-origin":"*","access-control-allow-methods":"GET, HEAD, POST, PUT, DELETE, OPTIONS","access-control-allow-headers":"X-Requested-With, Content-Type, Authorization, Date","strict-transport-security":"max-age=15724800; includeSubDomains"},"request":{"uri":{"protocol":"https:","slashes":true,"auth":null,"host":"gitter.ems.host","port":443,"hostname":"gitter.ems.host","hash":null,"search":null,"query":null,"pathname":"/_matrix/client/r0/createRoom","path":"/_matrix/client/r0/createRoom","href":"https://gitter.ems.host/_matrix/client/r0/createRoom"},"method":"POST","headers":{"Authorization":"<redacted>","Content-Type":"application/json","content-length":117}}}

Follow-up to !2313 (merged)

Part of #2609 (closed)