Begin splitting authorized_viewers form into roles and issues

The current form makes it very unclear and I hope refactoring a bit will
make it easier to simplify things, leave JS code under a static file,
etc.
Sort visibility roles in issues/show partial
Move javascript code related to the roles form into the static .js file
Rename 'involved-roles' css id to 'involved-roles-form' so that it's clearer and won't be mixed up with the '.involved-roles' css class