Use a .plist to install the app.
Just an idea, but you all have a certificate right? Why not just use your service to distribute the iOS app?
Just make a folder, or subdomain, like "install.protonmail.ch", upload a compiled version of the app, and make an install .plist. Reference the .plist on the site, and Tada! The app's installing!
Idk. Just an idea. :/