Depends on #6101

From the linked PR:

One feedback we have gotten on the app is that the QR code is rather large and consequently sometimes difficult to scan.

Based on the discussion in the previous PR, I propose this format:

{
    "i": [2, "indico.cern.ch", b64(checkin_secret)]
}

• Since we need to stick with a dictionary, let's use the smallest possible key "" (edit: we use i as a key)
• All data go inside the array, starting with the version
• If the base url uses https, we strip it
• Checkin secret is base64-encoded (saves 12 bytes)

Using indico.cern.ch as an example, we go from 178 to 49 characters, ~28% of the original size.

For accompanying persons, we could simply append the (base64-encoded) person id which would give us 76 characters (177 with the old format) so ~43% of the original size.

Visual comparison: