Add Element X iOS entries in the apple-app-site-association.

[pixlwave]

This PR makes 2 changes:

• Add Element X app IDs to Element's client definition.
• Tidy up the generated site association file
  • The empty apps array isn't expected for applinks definition so I removed it.
  • The webcredentials definition is to support password filling on the site, which as far as I understand it, matrix.to doesn't have any login form, so I removed this too.

The generated file looks like this after these changes:

{
  "applinks": {
    "details": {
      "appIDs": [
        "7J4U792NQT.im.vector.app",
        "7J4U792NQT.io.element.elementx",
        "7J4U792NQT.io.element.elementx.nightly",
        "7J4U792NQT.io.element.elementx.pr"
      ],
      "components": [
        {
          "#": "/*"
        }
      ]
    }
  }
}