Make the group audience field visible in the form display

[pfrenssen]

This is a fix for #144.

When the group audience field is added to a bundle (e.g. by creating a group content type) then the field is hidden in the form display. Most of the time the site administrators will want to have this field visible, because it is needed to select the group the content should belong to.

How to test manually

• Enable the field_ui, node, og and og_ui modules.
• Create a new content type at admin/structure/types/add. Name it "School", and select it to be a "Group" in the "Organic Groups" vertical tab.
• Create a second content type at admin/structure/types/add. Name it "Class".Select it to be "Group content" in the "Organic Groups" vertical tab, and set the target bundle to "School".
• Now create a new node of type "Class" at node/add/class.
• You should see the Group Audience field.

[pfrenssen]

This is tested in #149.

[RoySegall]

If this is already in #149 I think we can close this one and merge the other PR.

@amitaibu ^^

[pfrenssen]

Sure! This was split off from #149 to make it easier to review, but it is indeed merged in #149 to simplify the test there.

Closing in favor of #149.