Ability to make a label a priority from edit page

Why?

When first starting to use labels I ended up on the edit page and couldn't work out how to make an issue a priority. I don't think this proposal is really needed as blank state improvements in gitlab-ce#20852 would have prevented me having got there. However in busy projects new users might still find this useful. Once someone has learnt how to do it here they would be more likely to use the stars on the labels index.

What?

The simple solution is just to have a star you can toggle. This would add the label to the bottom of the priorities list. Since there is more space on the edit page it could have a description to make it clear what the star is for.

A more complicated solution would be to allow changing order too, but I think that's unnecessary.