Better monospace font for wiki textarea

Currently our CSS specifies just 'monospace', which uses the browser's default monospace font. This usually isn't the nicest-looking monospace font available on the platform.

We should add some styles which use the monospace fonts used by the OS on each platform:

• On Windows, Consolas
• On Mac OS X, Monaco.

(Any recommendations on an appropriate Linux monospace font would be great! )

The font size needs to be specified in px (rather than the current points) because in my testing these fonts appear to vary considerably in size when the same point size is specified. Pixels don't have the same problem.