List matches from different bill versions in bill-to-bill

[aih]

We have de-duplicated bill-to-bill section matches. However, many of these duplicates come from different versions of the second bill. In the display, these can be collected into one row, and add one column for 'Version'. In that Version, column, here, for example, we'd have ', IH, EH, RH' (with '' for none). This is because section 1 of 114hr2209 matches section 2 of 115hr1624 four times, for different bill versions. (This is not a top priority, since the basic bill-to-bill view currently works and is accurate).

[aih]

Resolved with #149. Only calculating bill_similarity for the last bill version.