Implement deploy status checks for github integration

[machisuji]

This introduces a new state for pull requests called 'deployed'.
A cron job checks the deploy status of every pull request regularly and updates the state accordingly once deployed.
It will also add a comment to each relevant work package to let users know where the pull request was deployed to.

To make this work an introspection permission is introduced and the APIv3 root endpoint amended with the deployed core SHA that is running.

Lastly, next to the deploy_targets table another table github_deploy_status_checks is created used to make sure the deploy status is only checked as often as necessary and not more.

To do

• implement deploy status job doing the actual work
• add deployed state in frontend
• add UI to allow configuring deploy targets
• add tests