Fix hardcoding of used Graph endpoint URL

[jimdigriz]

Fix some missed plumbing.

Whilst doing so, allow the token refresh to support providing an origin header too.

[esabol]

To be honest, the Origin commit should be a separate PR, imho. I also think there are more changes needed to properly add a davmail.properties parameter than what you have here, but I'm not sure. Usually new parameters are added to src/etc/davmail.properties. Might need to add it to some documentation files too.

[jimdigriz]

Origin removed from this PR, it will come in another.

[jimdigriz]

Added some entries to src/etc/davmail.properties as requested for davmail.graph{Url,Prefix}.