added ajax_resync_items() for g+

count set to 99999, because i found no way to count g+ post ahead
strat for an implementation of ajax_resync_favs

* i can’t do the javascript stuff because of limited capabilities. but
on the php side, i think this might work: add an type argument to
ajax_resync_items that either loads posts or favs.
* also a missing translation for the instagram mod
* missing: count_items() for favs