First implementation of topic loading from JSON

WIP

Coding the new topics page by hand is infeasible, given that we have
many sections each containing many articles. We'll want to automatically
generate HTML by reading some configuration file - all topic info is
placed in /data/topics.json. This is read once and used for generating
the page display.

Note: the code reading the topics file is currently a massive hack. The
`global` keyword wasn't working, preventing us from doing:

  $topics = getTopics();
  function renderTopics() {
    global $topics;
    // etc...
  }

Instead, we're forced to use a dirty hack with the superglobal $GLOBALS
variable.

/data/topics.json is currently a skeleton - it contains entries for
article categories (which equate to the links on the current topics
page), but the individual articles within each section aren't added. We
may want to write a script or something to manually extract article
names/urls as it will be an extremely time intensive process otherwise.