Make slight style changes

$var for jQuery variables, conditional operator improvements, cache
$this
{{escaped}} macro placeholder escapes markdown characters in selection
{{escaped}} also escapes []()
Use https resources if accessing reddit over https.

Expandos still need to be updated for sites that support https.
Use https when necessary for styles, too
Force HTTPS on all images

// on Firefox translates into resource://, not http(s)://

Some of these are not strictly necessary (mainly the img src URLs
and the CSS that we insert in res_user.js), but there's no harm in
using all HTTPS