Abstract article list class and midsection streamfield on homepage

[SamuelmdLow]

For events readers want to follow (like the Palestinian solidarity protests or AMS elections), it would be nice to feature stories about that event together on the homepage.

I think we should rename the links stream field to midsection or something. That would then contain the links as well as these featured topic blocks.

These featured topic blocks would be very similar to the section page preview as well as the sidebar previews. Since they share fields I think they should inherit from a shared abstract class that has

• template: Choiceblock for a template selector
• num_of_articles: Integerblock to determine how many articles to retrieve
• link: UrlBlock for linking to the page where all articles of this type are listed (this wouldn't be included in the panel of any inheriting
• title: Charfield