Add issue to epic, from issue

• A customer has requested to be able to add an issue to an epic, from the issue itself.
• I think it makes sense as a use case. But I'm not sure how the design would look.
• If we make the issue sidebar have the ability to add an epic, should it be a dropdown field where you can search/autocomplete and find epics? Or would you just paste in the epic link? If you would have the epic link already, then why don't you just go to the epic already and make the association there?
• In this issue, we should also update the issues API so that you can get the associated epic from an issue and also set it.

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