specify icon for admin sections

[christophemacabiau]

Description of changes

specify icons for admin sections

To specify an admin icon, use an object (you will find the icon name here):

keystone.set('nav', {
  posts: ['posts', 'post-categories'],
  galleries: {icon: 'rocket', paths: ['galleries', 'enquiries']},
  users: 'users'
};

Related issues (if any)

#4382
not looked sufficiently enough for a similar issue (wont do it again)
this pr is slighty different + doc

Testing

• Please confirm npm run test-all ran successfully.
  9 tests failing with mongodb, I dont see how this could be related to this commit