Edit Message dialog: add option 'Signed-off-by' signature

Hi, I contribute to a few projects that require the "Signed-off-by:" footer in the commit message. 

I'm forever forgetting to add it manually though, so have to ammend commit messages to add it, force push, etc.

While I know there's an option to add this in the Local->Commit dialog box, I always use the Commit pane in the main window which doesn't expose this option.

Also, the "Edit commit message" dialog box also does not have an option for this, and it's tedious to type it manually (without typos).

It would be great if there was a per-project setting to always add this (similar / next to to the options to add GPG signing to commits)

Separately, it'd be handy if the checkbox to add this was also added to the "Edit commit message" dialog.

Cheers,

Andrew