Fix logic for updating cache and setting variables.

Turned (sx-network--ensure-user) into (sx-network--initialize) and added
as hook for initialization.
Correct naming of cache to correspond to sx-cache-get.
Replace (sx-favorites--ensure-favorite-list) with (sx-favorites--initialize).

Simplify initialization since it is run at startup rather than tested
for by functions.

Clean up (sx-favorites--update-site-favorites) to provide the same
values for new sites as for existing.