Added nasty web preview iframe to front-end

This is purely to demonstrate calling up the previewer from the
front-end. It needs a lot more work to integrate!
Simplified URL of previewer servlet
Added bower.json to replace bundled AngularJS
Now using SCSS instead of raw CSS
Trying out angular-ui/bootstrap
Started on initial GUI layout & styling

This follows from Balsamiq protopying done earlier today, which is now
on the dev wiki
Minor tidy of BaseController

* Tree model stuff now lives inside $scope.treeModel
* Added dev/debug button to dump various state objects to console
WIP: Front-end preview imlpementation
Rendering key tweak for web previews

I've added a key to hide the content control panel, which will now be
set by defautl when showing WEB previews in the front-end. This cuts
down the number of buttons clumped together in the GUI, which may have
been confusing for users.
Now supports preview of various node types
Moved previewer HTML generation into directive
Tidied up previous few commits

Front-end node previewer is now working reasonably well.