Add a setting to allow anonymous access to individual gists while still RequireLogin everywhere else

[lf-]

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 https://github.com/thomiceli/opengist/issues/228.