make "Disable editing others comments" a configurable option

[zzkt]

The discussion at #125 covers various options for enabling and restricting editing of comments. I had assumed undeletable comments were a bug/regression but looks like there are good reasons and different uses cases for both strict and open editing (eg #201)

versions before 0.1.83 (possibly?) allowed comments to be edited and deleted by anyone with permission to edit the pad.

the changes @ilmartyrk introduced with /pull/163 ensure comments can only be deleted by the original author and the session must still be active from when the comment was created.

with the permissive mode, anyone can edit or delete comments. With restricted mode, there is no way to remove or edit old comments from the web interface (changes can only be made in the db). Either can be considered a feature or a bug depending on the context.

maybe a config option?