allow option for requiring description when editing page. This is so if a commit to an rcs is used, the commit message will not be blank.

> Duplicate of [[todo/Allow_web_edit_form_comment_field_to_be_mandatory]] where
> Joey indicated that he didn't want this in ikiwiki core, but would
> accept a plugin that did it.
>
> Expanding on what Joey said there a little, the problem I have with
> *requiring* a commit message is that solving a social problem
> by technical means rarely works. If you can't persuade users
> to obey a policy like "provide a nonempty commit message", then
> you can't persuade them to obey a policy like "provide a *useful*
> nonempty commit message" either. I used to work on a project
> whose Bugzilla had been configured or patched to require a comment
> whenever you changed a field (e.g. priority, cc, ...) and in
> practice that just led to a lot of wasted time when people tried
> to triage bugs quickly, and a lot of comments whose text was
> ".", " ", or on at least one occasion, ☃
> (U+2603 SNOWMAN).
>
> If your chosen RCS has a technical constraint that the commit
> message must be non-empty (and will just not work otherwise),
> that's another matter; I'd say that in that situation
> it's appropriate for its plugin to replace empty commit
> messages with "." or gettext("update") or something. --smcv