Facebook: support building database of wall posts from scratch

[micahflee]

When building these, we'll need enough posts in a test account to support pagination, or infinite scroll, or however FB does it. For people with a lot of data, they may need to sit there for some time while Cyd scrolls to the very earlier post.

Like with #354 for X, we should capture rich data about each post: all info we can about media (both images and videos), links, mentions, etc., which may be useful for a future migration feature.