Change visualizations varnish keys: prefix using database name

When invalidating users cache we need to invalidate all his cache, including
vizjsons, embed maps, tables and so on.

Vizjson varnish keys are now prefixed by the user's database name, this way
those objects will be invalidated when calling User#invalidate_varnish_cache

cc @javisantana
CDB-1521 #resolve Invalidate user cache after changing his account type