Allow admins to make other users admins

:fork_and_knife: Admin users can now navigate to a users profile (via the URL) and make another user an admin. Once the user is an admin they are indicated as such with (Admin) next to their username on their profile.
Can we use Pundit to hide the details of when the current user can make the given user an admin? I think ideally this conditional would be something like:

```ruby
if policy(@user).make_admin?
# ...
end
```