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.
Update installation-vagrant.rst
Merge pull request #2207 from ezio-auditore/patch-1

Update installation-vagrant.rst
Merge pull request #2205 from openjck/update-bugzilla-link-again

Link to [good first bug] bugs, not [mentor] bugs
bug 950945: Fix an error with waffle while attempting to generate sphinx theme layout
Merge pull request #2209 from lmorchard/fix-sphinx-theme-generation

bug 950945: Fix an error with waffle while attempting to generate sphinx theme layout