Allow using external index.json and stickerpack

[p1gp1g]

The idea behind :

• Give a public instance of stickerpicker users can use directly. Users will have to create a github page with an index.json. It is then possible to have a repo stickerconf to fork with default stickers.
• Share more easily packs (Why not having a public repo where users can publish their pack, with a preview webpage)

If there is no config GET parameter, it does today's behavior (index.json @ /packs/index.json).

The packs in the index.json takes the full URL

{
  "packs": [
    "https://example1.tld/stickerpicker/web/packs/aaa.json",
    "https://example2.tld/stickerpicker/web/packs/bbb.json",
    "https://example.tld/stickerpicker/web/packs/ccc.json"
  ]
}

• Example of conf : https://p1gp1g.github.io/stickertest/
• Example of widget URL : https://p1gp1g.github.io/stickerpicker-dev/web/?config=https://p1gp1g.github.io/stickertest/

Note: json packs and index.json needs to have wildcard CORS

[p1gp1g]

For the public repo, I've done a preview feature here : https://github.com/p1gp1g/stickerpicker/tree/preview It currently lists available packs in the config. Clicking on a preview copy the packFile URL. It misses (at least) :

• setting for a max previews per page
• previewing all the stickers in the pack

I'll open a PR for the preview when/if this current PR is merged. Contribution is welcomed :)