Facebook export: handle group posts

[redshiftzero]

Followup to #405
Related to #380

Import group posts from the export file and display them on the generated static site

[redshiftzero]

Will be needed for #406

[redshiftzero]

The export has group data stored in:

• your_facebook_activity/groups/group_posts_and_comments.json (despite the name, this only contains the posts AFAICT)
• your_facebook_activity/groups/your_comments_in_groups.json

For each group-related item, there's a group string, and a timestamp.

There is a group fbid in your_facebook_activity/groups/your_contributions.json but not sure what that maps to yet