Fixes #4525: Decouples group content visibility from membership policy

This succeeds #276 (and #233), rebasing the previous work and adapting it to use the recently-added GroupItemVisibility component. 
* This should implement the item visibility rules shown in the [spreadsheet of doom](https://docs.google.com/a/elgg.org/spreadsheet/ccc?key=0AlAO5SiXoBNGdExuTjgySk1xeDdubm04dmZHU01WQVE#gid=0).
* To group edit form adds "Accessibility of group content" dropdown
* To content edit form adds notice that's always displayed when user has selected an access level that will be overridden by group policy
* Adds basic tests for ElggGroupItemVisibility
* On the group profile, displays several different messages when content is inaccessible for some reason
* On group edit form, properly encodes user names on the dropdown to transfer ownership 
* Adds Elgg_UserStatus component (mainly for unit testing purposes)

The minimal tests pass, but obviously this will need some serious testing.