Make topic lists show last activity more intuitively

Apparently editing the most recent (and only the most recent) post in a topic [causes the topic to be bumped](https://github.com/discourse/discourse/blob/master/lib/post_revisor.rb#L56), which can be confusing since the displayed date on the topic list is the last *post* date.

I tweaked things a little so the last activity date will be displayed as the `bumped_at` age when appropriate, and the tooltip will contain both pieces of information. The last post date is now also shown when the topic is bumped even when there are no replies (i.e. the topic post itself was edited).

I tried to avoid confusion by reducing the opacity when the value for the second column is due to the bump instead of to a new reply being posted by lowering the opacity, like in the following screenshot:

![tooltip for new activity](https://f.cloud.github.com/assets/449006/166156/5ea325bc-798c-11e2-8bfc-e8336a52f50d.png)

I'm not positive this is the best solution, but unless bumping on edit is dropped completely the current behaviour seems more confusing.