Scroll with pinned headers on long tables

Please merge #275 first and I'll rebase this branch against master.

This pull request makes long tables not-quite-so-long and introduces the same `overflow: scroll;` behaviour that we have in Limelight, but with much less code and much better markup :cocktail: 

Unfortunately it means we need to render the entire table module again client-side, which makes me a bit sad. But oh well.