Add container group ACL to access options for embedded files

[mrclay]

Embedding an image in a group-contained object's description field results in a file owned by the uploader and contained by the uploader. This is appropriate, however a user may want to make the image only accessible by group members (and in the case of arbitrary group items, this might be a good default).

Currently, the containing group ACL is not available in the access list for the uploaded item; the uploader has to make the image accessible to at least all logged in users to have it visible by group members.

I understand that, typically, user-contained items should not have groups in their access dropdowns (to avoid cause confusion), but I think this is a use case for an exception.

This seems like a messy problem to solve, but I thought I'd document it anyway.

[cash]

Best solution is probably to pass the container guid to the embed plugin so that it properly sets the page owner. Once that is done, it should be easier to set the access list and anything else that may be group specific.

I'm setting this to the 1.8.2 milestone as it would be nice to add this early on with Elgg 1.8

[cash]

My solution puts the file in the group rather than putting in the user's container. Any arguments against that?

[Cash Costello]

Fixes #4086 handling containers for embed uploading

Changeset: 3d8ca76d4b8b9e69a92f5f5b034609b410c9ee8c