Better extension handling

[trickypr]

This is primarily an architecture issue, but we need to handle extensions better. Most of these changes will be made inside of gluon, but have a large impact on the browser. The following needs to change:

• [x] Static urls should be avoided, instead a platform, extension and version should be provided. Platforms:
  • [x] AMO
  • [x] Github Releases
• [x] @fushra-robot (or maybe a bot implemented using probot) should file update issues and maybe pull requests
• [x] Update manifests should be generated and hosted on updates.pulsebrowser.app

[trickypr]

New gluon.json prototype for my future reference:

{
    "addons": {
        "ublock": {
            "platform": "github",
            "id": "uBlock0@raymondhill.net",
            "repo": "gorhill/uBlock",
            "version": "1.43.0",
            "fileGlob": "uBlock0_*.firefox.signed.xpi"
        },
        "tabliss": {
            "platform": "amo",
            "id": "extension@tabliss.io",
            "amoId": "3940751",
            "version": "2.6.0"
        }
    }
}