Check sessionURL against jsbin.root

This stops us from trying to load in from SessionStorage when we're
refreshing on the homepage.

Fixes bug where preprocessors in templates wouldn;t work when clicking
new on the homepage
Don't save on exit when clicking the new button
Check panels.saveOnExit is not false

The reason we don't use coercion here is because we only want to abort
the save if we've explicitly set the saveOnExit value to false
Reformat if - jshint
Don't set panels.saveOnExit to false

This is because that has had no effect since 2012-05-14 so introducing
a check against the variable now could cause unwanted and unexpected
results.

Although this commit removes a whole fork of code, the variable being
set was unused and unreferenced before. So the functionality is the same
as before.
Merge pull request #1715 from jsbin/fix/preprocessor-template

Check sessionURL against jsbin.root