More dynamic activity scores

This introduces a more dynamic system of activity scoring, to improve
the way bubbles "bubble up" due to their activity. There are a few
different parts we can tune here, and it's likely we'll need to make
adjustments once we get a feel for how this works in practice.

The basic idea here is:

- We assign points for certain types of event that happen on a bubble. A
  boost gets 1 point, a comment gets 10 points, and so on.
- These points decay over time, at a rate of 50% per day. So old
  activity is worth much less than new activity. Bubbles should rise up
  quickly when acted upon, bit will float back down if left idle.
- Some comments can score higher than others: the first comment from
  each person on a bubble is worth more (20) because it signals that
  more people are getting involved; and comments that follow a comment
  by a different author are also worth more (15) because that signals
  there's ongoing conversation between people, not just a series of
  notes being left by one individual.

In terms of implementation, we persist the score on the bubble
whenever it changes, but we handle the decay on the client side. That
allows us to cache the bubble representation without having to
continually change it while its activity decays.

We also keep a separate `activity_score_order` attribute on the model.
This can be used to sort the bubbles in order of "most active", without
having to think about the decay.

test/test_helpers/change_test_helper.rb

@@ -0,0 +1,8 @@
+module ChangeTestHelper
+  def capture_change(target)
+    before = target.call
+    yield
+    after = target.call
+    after - before
+  end
+end