edit-testrun.js: Update table after "Failure reason" or "Notes" is edited

If "Failure reason" or "Notes" is changed in a test run's page using
`instaweb`, index.html has to be reloaded to see the updated values,
which is quite inconvenient.

This commit modifies edit-testrun.js to update the table with the
actual values straight away.

It uses the `truncate` function from sugar.js library to match the
functionality of `truncate` jinja2 filter on the server side;
see http://sugarjs.com/.
instaweb: Expire pages sooner to get the latest content

The current `instaweb` implementation has the following issue:
1. Change the value of "Failure reason" or "Notes". The test run page
   displays the new value as it's updated by the client side script.
2. Open another test run page.
3. Open the recently updated test run page again. The old value is
   displayed because the page is loaded from the browser cache.

Expire pages in 1 second to make sure that the latest content
is displayed in every case.