[5.x] Changes to User role methods

[duncanmcclean]

This pull request makes some changes to the role methods on Statamic's User classes:

• The ->roles() method now returns roles assigned via groups, in addition to roles directly assigned to the user.
• The ->roles() method is no longer a setter, only a getter.
• Adds a new ->explicitRoles() method, which returns the roles explicitly set on the user.
  • This method is also a setter, so you can do ->explicitRoles(['role_1', 'role_2']).

Breaking Changes

• The $user->roles() method, $user->hasRole() method and {{ user:is role="role_1" }} tag now check if a user has the provided role via groups.
• If you were setting a user's roles with the ->roles(['role_1']) method, you should now use the ->explicitRoles() method.