Accepting an invite to a room on another HS returns a 404

This issue has been migrated from [#2807](https://github.com/matrix-org/synapse/issues/2807).

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### Description

In the matrix-appservice-slackbot, the bot will auto join any rooms it has been invited to. If a user from another HS invites the bot to a room that hasn't been synced to the bots HS, synapse will throw an error `2018-01-18 01:26: [-] POST https://matrix.giveth.io/_matrix/client/r0/join/!TAeakIVVBKXyGipXoY%3Amatrix.org AS) HTTP 404 Error: {"errcode":"M_UNKNOWN","error":"No row found"}`.

### Version information

- **Homeserver**: Was this issue identified on matrix.org or another homeserver?

If not matrix.org: giveth.io
- **Version**: 26
- **Install method**: package manager
- **Platform**: VPS w/ debian 9