Custom OG_Vocab widget

Only display the vocabularies of the group this node is attached to. In the node creation form we don't know this yet, so there are no vocabs shown there.

Note: When adding content within a group (purl path <group>/node/add/<content type>) the vocabs of this group are shown. However, there is a selectbox to manually select the group for this content. This selectbox will disappear in the future so this won't be an issue. Now you can select a different group, while the vocabularies are fixed.