Use a GitHub app for sync-team

[marcoieni]

Right now to apply the changes, we use a classic PAT.
We want to create a GitHub app with write permissions and install it on all orgs.

Question to be confirmed:

• is it possible to install a GitHub app with write permissions for all repositories?
• can we use merge-bots with a GitHub app? The read only github app can't read actors that are allowed to push. e.g.:

[marcoieni]

#2255 added the ability to install github apps with a github enteprise app

The write.rs file now assumes that installing and removing an app is done with a PAT.

To implement this issue, the installation needs to be done via a GitHub app.

[Kobzol]

That sounds possible with the current GitHub API, I'll work on it after #2255.