myria-web: report concise runtimes where appropriate

E.g., in infoboxes about operators, in the network view, etc.

The only place we need detailed times is in the gantt charts and x-axes.
networkvis.js: UI fixes for sender

1. The average number of tuples can be different on the source and
   destination sides, when different numbers of nodes are involved. We
   were using the destination average as the source average. Fixes the
   second half of #135.

2. The average line was a little bit short on the source side, because
   its height was set by the matrix width not the matrix height. Fix this.
Merge pull request #145 from uwescience/network-viz-fixes

Network viz fixes