[Feature request] Email notification when (last) editor declares conflict of interest.

[TobiasKappe]

Is your feature request related to a problem? Please describe.
The conflict-of-interest (COI) system currently allows editors to declare a conflict when first opening an article. We are considering to use this at LMCS. Right now, seems that when an editor declares a conflict, they are removed from the article, but nothing else happens. This carries two risks:

1. Fellow editors also assigned to the article may not be aware of this conflict, and thus silently assume that somebody else is managing the article; or
2. When the last (or only) editor declares a conflict of interest, this could put the paper in limbo.

Describe the solution you'd like
We think this can be resolved by implementing two separate notifications:

1. When one editor declares a conflict of interest for an article, the other editors should be notified via email.
2. When the last editor of an article declares a conflict of interest, the editor in chief should be notified via email.
   This will help prevent articles from getting stuck in the process with nobody to care for them.

Describe alternatives you've considered
We could manually patrol articles that lose their editors, but this seems like an error-prone process as we would have to keep track of which articles had an editor assigned to them at some point.