Group delete error 500 when processing large amount of contents/members

Problem/Motivation

When deleting a group containing a large amount of contents and/or members, the time for processing all of the group_contents deletion in the preDelete() function may lead to error 500 due to server max execution time hit.

Steps to reproduce

• Create a group with 2000 contents & 2000 members
• Delete it via the interface

Proposed resolution

Split delete operations (especially group_contents entities deletions) into batches, to avoid hitting the server max execution time.

User interface changes

Users will now be able to follow the delete operations progress thanks to the Batch API.