[flags] Activate flag via URLs

[ptbrowne]

I think @enguerran has done something similar for contacts.

http://bank.cozy.tools:8080/?flag__toto=salut could activate the flag { toto: "salut" }

[enguerran]

https://github.com/cozy/cozy-contacts/blob/73c26e5f6aac408dd1fac2517760314f730b7268/src/components/Toolbar.jsx#L8-L10

const searchParams = new URL(window.location).searchParams
const fakeintentEnabled = searchParams.get('fakeintent') !== null
const vcardEnabled = searchParams.get('enablevcardimport') !== null