Support adding servers when users already have the app

[ghost]

** Discussed with Ben, Vinicius and Cassidy **

If the user DOES NOT have the uProxy app installed (the majority of cases), the link that we will ask them to click through will take them to the Play Store, and the server will be automagically added to the app by virtue of the referrer data passed along to the Play Store.

If the user DOES have the app installed, we need a way for the user to add the server to it. The link mentioned above will not work because they already have the app. Right now the app doesn't have a way to add servers to itself. It's on those instances that we would like to have a link that actually does the magic of adding the server to the app.

Phase 1: Support users who are on mobile devices
We want the access code to be coded on a "uproxy://" prefix, so that hopefully it will not be opened by the browser and will instead be managed by the uProxy app. This covers users receiving the invite code through instant messaging applications, SMS or email (provided they open the email on their mobile device). We will message users that they can only open this link when on their mobile devices.

Phase 2: Support users who are on desktop
For them, the uproxy:// link will clearly not work. Instead, for those users, we we want to display a QR code. QR codes will entice the user to grab the phone and point it towards the computer. The QR code will translate into the uproxy:// url mentioned above -- but since at that point it will be opened on the mobile device with the app, that will work.

Sidenote on Phase 2: Right now, given the length of the invite link, generating a QR code for it means having a rather large and detailed image. Prior to launching a QR code approach, we will probably want to have worked on shortening the invite links.

[ghost]

BTW, Ben found some tentative evidence that there is some CSS wizardry that can be done to get emails to 'dynamically' show different content (a uproxy:// link vs a QR code) based on their device on which they are being opened.

[fortuna]

FYI, this is also needed for users that install from the APK or other stores.

[fortuna]

One other issue I found: if the user clicks on the link, and does not pick the uProxy app, then it will open the browser.