implement new URL for public profiles

- caffeine.models.User:
  - use reverse in get_absolute_url
- caffeine.templatetags.caffeine:
  - simplify publicurl template tag
- caffeine.urls:
  - add url pattern "public" for caffeine.views.PublicProfileView
- caffeine.views:
  - extract base class PublicProfileView from ProfileView
  - redirect from ProfileView to PublicProfileView if request parameter 'u'
    is specified to keep compatibility with old URLs
  - separate get_context_data into public and private parts in
    PublicProfileView and ProfileView
- caffeine_api_v1.views:
  - use new public URL in random_users
- templates/includes/profilelink: use public URL
- templates/profile.html: use public URL for link
fix template error in password_reset_confirm.html