Workshops: Add a block for outputting a list of upcoming social learning spaces

[coreymckrill]

WIP

Refs #145

[hlashbrooke]

Since we are now using the Meetings calendar plugin, all events (now named "Social Learning Spaces") are actually stored on the site in the meeting post type, so this won't require any external API calls and can be gathered from local data. That should make this kind of thing a lot easier to do!

[hlashbrooke]

Opened #280 to update the social learning spaces to use the meeting post type instead of meetup.com events.