Fix group work items for RSS feed

MR: Fix group epics RSS feed (!211909 - merged)

Problem

Currently the Subscribe to RSS feed option on the group_issues and group_epics pages only returns project level work items.

This is evident from this:

1. _work_item.atom.builder which builds the rss feed. It always calls the project_work_item_url

2. work_items_collections which is responsible for retrieving the work_items for the RSS feed. But here we always set exclude_group_work_items to true which in turn only retrieves project level work_items even in the group level context.

We should fix this to return the correct group work items in the RSS feed where appropriate.

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Additionally we need to ensure that the RSS feed link includes either type=issue or type=epic when called from either group.issues or group.epics page, so that we only show the appropriate work_item type on each page and not just all work_items. This functionality is currently missing.

Acceptance Criteria

1. Call group_work_item_url in _work_item.atom.builder when RSS feed is triggered from a group issues/epics/consolidated_list page.
2. Set exclude_group_work_items to true in work_items_collections.rb only when called from group.issues page
3. Add the appropriate work_item type [issue, epic] to the RSS link filter params frontend