Github integration: repo transferred to organisation no longer triggers bot nor can new repo be added

### Description

I transferred a repo from cmcaine/tridactyl to tridactyl/tridactyl and the github notification bot stopped talking in my channel.

Checking integration panel, cmcaine/tridactyl is still present in list of existing webhooks but not in the list below of repos available to add and change notification options for.

tridactyl/tridactyl and repos from other organisations for which I have access are not listed as available options to add hooks for.

Checking the repo settings on github.com, the hook is still active and has been sending updates to the same API endpoint without interruption.

Possibly related? vector-im/element-web#3478

### Steps to reproduce

- make a github repo
- add github integration bot using riot-web
- transfer repo to an organisation you own

I expected the integration to continue despite the transfer and for the bot to continue to notify the channel about issues, commits, etc.

Log: not sent?

### Version information

- **Platform**: web (in-browser)

- **Browser**: Firefox 61
- **OS**: Arch Linux
- **URL**: riot.im/app

Start in the Riot web GitHub integration panel and reproduce the transfer from a personal account to an organisation. Check how the existing webhook and the GitHub API endpoint are handled after the transfer, including whether the new repository appears as available. Done means notifications continue and the transferred repository can be added or configured.

Written by the indexing model from the issue text.