Require obligatory prominence reason for reduced-prominence content

[WilliamWDTK]

>5 characters of text on requests and messages (but not annotations) which are reduced prominence, perhaps excluding back-paged requests.

[garethrees]

+1, agree with this, and also making sure that there's a reason when changing prominence of entire request (https://github.com/mysociety/alaveteli/issues/6746)