Remove project webhook when removing a project from the dashboard

[gtsiolis]

Problem to solve

Currently, when a user adds a project in a team through the dashboard for a GitLab repository, a webhook is installed. See relevant docs[1] for enabling prebuilds.

However, when a user removes a project from a team in the the dashboard the webhook is left behind.

Proposal

Let's remove the project webhook when removing a project from the dashboard for GitLab repositories.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gtsiolis]

Still relevant!