gist: make it a bit more clear how gist_access_id is used ... and how it is different from gist_id

A gist has a gist_access_id which gives access to it. For private Gists, it is
a multi-letter secure random string.

gist_id is the primary key in the database and thus an automatically
incrementing integer. It is also used as the not-so-secret gist_access_id for
public gists.

This gets rid of one odd safe_unicode applied to an int.