Show warning when overwriting existing Epic link

Problem to solve

If an issue already has an epic assigned, it shows that on the Issue Page on the sidebar, and you are unable to change the epic from that location.

However, if you are on an Epic Page, you can add Issues to it directly. If those issues already have an epic assigned, it overwrites that epic without notifying you of that overwrite (or even noting that it had an extant Epic in the first place).

Additionally, when on the Issue Page, you see that the epic was changed, but not from what (though you can see that in the history).

Proposed solution

Show a warning before Epic assignment - When an issue has an epic already assigned, if a user attempts to assign a new epic to it from the epic page, a warning should indicate that the issue already has an epic assigned, and what epic it is.

Show the previous state in the activity message - When an epic is changed, indicate both the new state and the previous state in the content of the activity stream. If there is a significant amount of activity between epic assignments, this makes it much clearer to the user what happened without them having to dig through that history.

Current activity message:

Possible update: