Add extra add buttons for global / instance badges

As admins often accidently created global instead of instance badges, we
now add extra buttons above the respective badge tables.

This is pretty ugly code, as controllers._available badges and the
corresponding templates would need refactoring which I don't want to do
now.
Merge pull request #708 from nidico/feature-add-badge-extra-buttons

Add extra add buttons for global / instance badges