Optimize e-mail notification templates

At the moment, e-mail notifications for the site are sent from Degrowth co-munity and have topics such as "[Group name] Gualter created some page".

Following the earlier co-munity version and the attempt to mimic a mailing list behavior (see also adding reply-to feature for email notifications), these should be changed to show the author name (but not e-mail address) and the subject should not include "user created" or "user commented", but just the node title and, on comments, the Re: before. For content updates it can probably stay as is.

If you have other suggestions on improving this user experience, they are very welcome.