improve quote reply selection

This improve the quote-reply selection based on Harald Eilertsen's feedback on [meta](http://meta.discourse.org/t/quote-reply-gets-in-the-way/1495/67).

The previous *algorithm* for detecting whether the selection was spanning two posts was based on the `.cooked` class which was too restrictive.

The new *algorithm* instead bases its detection on the `.topic-body` class with a slight edge case for the first post (which has a summary).

I've also taken the liberty to refactor some CSS into the `unselectable` mixin and applied it wherever it improved the selection.

This has been tested on:
- [x] IE 10 (Windows 8)
- [x] FF 20 (Windows 8)
- [x] Chrome 26 (Windows 8)
- [x] Safari 6 (OS X)
- [x] Safari (iOS 6.3.1)
- [x] Chrome (iOS 6.3.1)

Browsers compatibility is *damn* hard! :triumph:

Note: @coding-horror, I checked [caniuse](http://caniuse.com/#feat=user-select-none) in order to provide the minimum prefixes, but this feature is not well supported...