Drag&drop issues and epics in the epic tree - support ordering between all objects

In https://gitlab.com/gitlab-org/gitlab-ee/issues/9367 we started with drag & drop support. Soon we realized ordering between different types (issues & epics) is not that trivial from backend perspective. Therefore that issue includes only ordering of issues and epics separately.

Scope of this issue is to add support for ordering both issues and epics as designed originally in the mentioned issue.

It will probably need a new model and a migration to move current ordered state.

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