Donation screen

We want to add the possibility to make donations towards VideoLAN

subtasks:

Add an entrypoint in form a DonationButton to the viewcontroller (#354 (closed))
Add a screen where the user can enter the amount he's like to donate (#355 (closed) )
execute and trigger the Apple Pay flow (#356 (closed) )
On succesful donation show screen raining cones (#357 (closed))
Add testing for this flow

Edited Jan 13, 2019 by Carola

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