Edit mailtemplates for the preprint repository (notification for editors after submission)

[mattiker]

Some mailtemplates at the preprintserver are hardcoded, like the one the editors get after a submission is finished ("A new preprint has been submitted to {name of the repo}: {article name}")

Solution: Make it a setting in the repo wizard or django to edit this mailtemplate.

** Tech notes** We should migrate those email templates to use the settings system. They should also appear on the repo wizard