Link to [good first bug] bugs, not [mentor] bugs

Maris informed me today that he uses [mentor] to tag bugs that need a
mentor, regardless of difficulty, and that he uses [good first bug] for
bugs that would be easy for volunteers to pick up.
Merge pull request #2205 from openjck/update-bugzilla-link-again

Link to [good first bug] bugs, not [mentor] bugs