Fallback keys can be provided by client devices to be used in case the
supply of one-time keys run out. The server will store one fallback key
per user, per device, per algorithm. The server will keep track of
whether this fallback key has been used or not.
The /keys/claim endpoint now provides a fallback key
if no one-time key is found
The /keys/upload endpoint now accepts fallback keys
The /sync endpoint now informs the client of the algorithms for which it
has an unused fallback key in stock.
