Guest mode – browse without registration

Implementation of feature request miguelfreitas/twister-core#143. This is intended to lower the entry barrier for new users, who can now start browsing right away, and decide to register after the UI tells them they need to log in to be able to use active functions like replying, posting, and following others. It can also be used to run public read-only servers, as Ira and Mpancorbo wanted.

To test it, you can either remove all user accounts from the testing instance, or run it on a freshly installed one. The guest mode is used automatically when no accounts are available.