Review Board will collapse old reviews to streamline information. 

While the data is visually hidden it will still be computed 

and loaded to the client which can cause slowness.
This change makes collapsed review entries get sent to the client 

empty, then fetch its content on request. When they expand the entry, we 

first check if this entry already has content loaded, if not then load it

by using the new function "fetchEntryUpdates" in reviewRequestPageModel.js

which can trigger an immediate update to load the content of an entry.

This function uses and extends the machinery related to "page pending updates"

In order to make it feel even snappier, I add a mouseover event listener 

on the expand button and the expand all button. We start to pull down information

before they even lift the mouse button.
Only load the entry content if the entry is of type "review". Without this restriction, 

when they click the expand all button, the entry of type "initial_status_updates" or 

"changedesc"also tries to check whether it has content which causes an error.

Created entryViewTests.es6.js that contains test cases for the

behavior of expanding an entry. All tests passed.
Ran all Javascript unit tests and no newly introduced test fails.