Refactored Likes module

Modified get_likes function to output more predictable HTML structure,
and rationalised JavaScript a lot – dropped brute force DOM
manipulation in favour of simple class substitution.

Posts can now be liked and unliked multiple times without requiring a
page refresh.

Like message is now displayed without clickable image/text for non-JS
clients – init function enables liking on page load for JS-enabled
clients.

Both “(Like)” and “(Unlike)” texts are always present in the DOM if
“like with text” setting is enabled – which one to show is now handled
entirely with a couple of CSS rules.

Removed some presentation-related CSS that shouldn’t be forced by the
module; link opacity, underline etc. should be controlled by the blog
theme.