Prevent jitter when loading lead section on slow connection.

Sometimes lead section content appeared to start halfway
down the screen when using slow connection. Was caused by
bottom scroll margin only being added after non lead sections
had completely loaded and how this caused the code which
limits vertical scrolling to get confused. Solved by
adding bottom margin - and last modified by and page
history footer - to end of lead section as soon as it
loads, and then injecting the non lead section content
above this footer stuff after it is retrieved.

Also added a couple of nil crash prevention checks.

Change-Id: I384896a8f2db25692b13794b970a199fba6d8089
Merge "Prevent jitter when loading lead section on slow connection."