Refactor sx-network--get-associated to avoid double call to
sx-cache-set.

Fix provides lines (missed quote on symbol).
Fixes as per github comments.

(sx-network--get-associated): Ensure user-sites is also set when
retrieving cache.
(sx-network--ensure-user): Simplify.  Used to ensure cache is loaded
before attempting to use it.

Fix quoted lambda in sx-network--map-site-url-to-site-api.