Add support for configurable padname suffix

[doobry-systemli]

It would be great to have a configuration setting that allows to set a padname suffix.

Background is that it's common for Etherpad instances to treat pads differently depending on the suffix. We plan to run a cronjob that deletes unchanged pads after 30 days per default, but pads with suffix -keep should be kept for one year after last change.

Certainly, pads that are created and maintained from a Nextcloud account, should be left alone. In order to identify those pads on our Etherpad instance, a (configurable) name suffix (e.g. -nextcloud) would help a lot.

[doobry-systemli]

I created PR #84 which implements this function. I'm looking forward to your review :)