Simplify method calls in projects/visibility partial

- Project#member_principals association is already defined to include
  principals
- sort{|a,b|a.foo<=>b.foo} is equivalent to sort_by{|a|a.foo} (with the
  added benefit of a cache that calls a.foo only once per object, then
  serves from cache...)
Don't reinvent 'role_ids_for_current_viewers' helper in issues/show_involved_roles partial
Work consistently with ids instead of strings in all views

+ introduce Role#authorized_viewer_ids for clarity