Style target notes for easy visual identification

When linking to a previous note in a issue discussion, can be hard to identify the linked comment, specially when the browser doesnt even scroll to it (cause it is already showed).

This adds a little CSS to visually aid the user clicking those internal links.

![notes-highlight](https://f.cloud.github.com/assets/517228/513711/41061f5c-be45-11e2-900a-b77ab2b750cb.png)

@randx Can u put this on the 5.2 final release?