Update ReferenceRewriter to handle projects and groups

What does this MR do and why?

Changes in this MR are intended to allow ReferenceRewriter to rewrite references in the markdown content while the content is being moved across different namespaces.

Previously we only had "move/clone issue" functionality. Issues used to be a project level only item. Now as we transition to WorkItem we also have group level work items, e.g. Epic Work Item, and in order to keep parity on "move" functionality we need to be able to also move group level WorkItems(Issues). So this MR adapts the ReferenceRewriter to handle references in markdown when items are moved across groups and projects.

As part of this improvement I had to also change the IssueReferenceFilter so that it does not passes on group level items, in order for all group level items to be handled as Work Item, but WorkItemReferenceFilter. Otherwise if references are detected by IssueReferenceFilter those are not being picked up by WorkItemReferenceFilter.

For spec coverage I have converted existing spec/lib/gitlab/gfm/reference_rewriter_spec.rb to ee/spec/lib/gitlab/gfm/reference_rewriter_spec.rb in order to be able to create group level work items as group level work items are EE feature. The EE spec cover cases of rewrite across Projects, ProjectNamespaces and Groups.

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