DRY up passing element bounding rects from UIWebView back to Objective-C.

* Centralize string parsing and offsetting in UIWebView method.
* Centralize element bounding rect formatting in util.js (né spoiler.js).
Don't ask to open URL copied from a PM.
Rename "Rendering" group to "Web View Junk".
Document AwfulActionSheet.
DRY up action sheet for long-pressing a URL.
DRY up handling the network activity indicator for UIWebView.

Not 100% sure about this implementation but it's finicky enough that
copy/pasting is not very helpful.
Preserve/restore scroll position in PM view.

* Move the scroll fraction get/set out to a UIWebView property.
Support font scale setting in PM view.