Add community contribution label to all PRs from community contributors

[gtsiolis]

Problem to solve

Community contributions somestimes fall between the cracks and go unnoticed for a long period of time. We also recently changed how we automatically close stale PRs in gitpod-io/gitpod#5898 which could lead to closed community contributions (PRs) without being noticed.

Proposal

If possible, let's highlight community contributions with at least adding the community contribution label automatically.

[svenefftinge]

Since we now automatically add team labels depending on the areas a PR touches, the responsibilities are clear IMHO and I'm not sure the community label would add any value in surfacing the PR.
Team members should build the habit of regularly (e.g. twice a day) checking the PR list for their team and see if they can unblock anyone (no matter they are team members or not).