fix apps/bookmarks if oc_bookmarks_tags is empty

If you have bookmarks, but there are no tags in oc_bookmarks_tags, the
query doesn't return any results. Using a left join fixes this.

Reference:
http://stackoverflow.com/questions/3171276/select-multiple-tables-when-one-table-is-empty-in-mysql



Signed-off-by: Florian Pritz <bluewind@xinu.at>