* Add a setting to allow accessing individual gists without auth
This is a middle ground between the existing setting "Require Login",
which requires login to do anything at all, and having it off, which
shows a public list of gists and more generally allows discovering info
about the users/gists of the instance without login.
The idea of this setting is that it is "require login" for everything
except individual gists.
Fixes#228.
Co-authored-by: Thomas Miceli <tho.miceli@gmail.com>
