FIX: Shift+Click to select text does not work in topics

Meta: [Shift-Click to Select Text Does Not Work in Topics](http://meta.discourse.org/t/shift-click-to-select-text-does-not-work-in-topics/6238)

This allows users to *extend* their selection using <KBD>shift</KBD>+click like so:

![shift-click](https://f.cloud.github.com/assets/362783/442038/458a6d44-b134-11e2-99e2-1bfe33b9b927.gif)

I also corrected a bug where the quote reply button would still be displayed when the user would *extend* her selection using the keyboard (eg. <KBD>shift</KBD>+<KBD>→</KBD>) or the aforementioned technique:

![hide-quote-reply-button](https://f.cloud.github.com/assets/362783/442069/5e742e84-b135-11e2-8d2f-2f896a914066.gif)