Inline edit of an issue body should prevent accidental navigation

1. Go to an existing issue
2. Begin editing the description via inline edit
3. Make changes
4. "Accidentally" navigate away from the page (two-finger swipe on macOS, for example)
5. Changes are lost

Once we detect changes made to the field, we should prevent navigation with a popup confirmation.

Losing changes is a really bad experience, and it's especially unexpected because we do save user content in non-inline-edit fields such as comment and new issue forms.

@EmilyKyle has lost work to this issue three times already this week.

cc @jschatz1