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. It's now a lot easier to see what the  JS and PHP are doing.

### Changed behaviours:
1. Posts can now be liked and unliked multiple times without requiring a page refresh.
2. Like message is now displayed without clickable image/text for non-JS clients – `init()` function enables liking on page load for JS-enabled clients.
3. 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.
4. Removed some presentation-related CSS that shouldn’t be forced by the module; link opacity, underline etc. should be controlled by the blog theme.
5. Tightened up CSS targeting: the module knows it's outputting a div, so we should be targeting "div.likes".