Add right margin guideline to commit message editor

This pull adds a configurable right margin to the commit message editor. See [IDEA-96737](http://youtrack.jetbrains.com/issue/IDEA-96737) for more context.

Definitely let me know if you have any questions/suggestions; I'd be glad to make any changes you folks would like to see to get this in.

(My signed contributor agreement should be in the contribute@jetbrains.com inbox)