Add description attribute to IStyle, expose it in REST.

Open username-removed-1178112 requested to merge J08nY/mailman:list-style-descriptions into master

Adds description attribute to the IStyle interface and exposes them to the REST api. This is to aid list style selection in Postorius(https://gitlab.com/mailman/postorius/merge_requests/214), so that the user has something human-readable to look at.