Detect wider community authors from www-gitlab-com data

What does this MR do and why?

This MR includes two commits:

• Introduce a TeamMemberSelectHelper.team_member_exist? method
• Detect wider community authors from www-gitlab-com data

That means we now use www-gitlab-com data to detect if an MR author is from the wider community or not, instead of checking membership of gitlab-org.

We discovered that we sometimes invite external contributors as Developer in the gitlab-org group, so the group membership data isn't accurate.

The www-gitlab-com data should be more accurate. Note that this data includes Core team members, so the TeamMemberSelectHelper.team_member_exist? exclude them, so that Event#by_team_member? / Event#wider_community_author? don't need to care.

In turn, this allows to greatly simplify some logic around detecting wider community contributors.

Expected impact & dry-runs

No change in behavior is expected, but more MRs should be labeled as Community contribution now.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review