Do not suggest running task news if there's no news to share

[djmitche]

Currently, if we make a release with no new news items, users will be reminded to run task news because the current version is newer than their news.version config. However, when they do so they will be told nothing is new. That's not a great UX - better to just not remind them if there are no new news items.

This will probably require moving the reminder out of CmdCustom.cpp and into CmdNews.cpp, which can calculate whether there are any news items to show. In fact, it's probably best to factor all of that out into a src/News.{h,cpp} which both CmdCustom.cpp and CmdNews.cpp refer to.

[djmitche]

Split from #3196.

[djmitche]

If 3.1.0 doesn't have any other news to share, we should do this in 3.1.0. I think at least the task purge support will have a news item, so this is probably OK.