Editors cannot assign themselves as Reviewers

[ajrbyers]

Is your feature request related to a problem? Please describe.
Currently editors are not considered valid reviewer candidates for an article they are assigned as an editor to.

Describe the solution you'd like
Allow editors to assign themselves as peer reviewers.

Describe alternatives you've considered
It should be consdiered that it is not appropriate for an editor to also be a reviewer for an article. Should Janeway be opinionated about this or should it allow this?

[StephDriver]

consider in relation to:

• Warn editors assigning a peer reviewer if that reviewer is also a section editor/editor #4144

[StephDriver]

make this a setting.

Alert @severett89 for comment / awareness.