More readable mapping.json

[hbons]

Let's go from:

{
  "add": 60000,
  "plus": 60000,
  "gist-new": 60000,
  "repo-create": 60000,
  "lightbulb": 60001,
  "light-bulb": 60001,
  "repo": 60002,
  "repo-delete": 60002
}

To something like:

{
  "60000": [
    "add",
    "plus",
    "gist-new",
    "repo-create"
  ],
  "60001": [
    "lightbulb",
    "light-bulb"
  ],
  "60002": [
    "repo",
    "repo-delete"
  ]
}

This will make it possible to see which names map to the same icon at a glance, and will make it easier to add/edit names.
We'll also need to update the export scripts.