Backblaze B2 External Storage

Add Backblaze B2 to supported external storage.
Backblaze is one of the least expensive storage providers and one of the most reliable.

Hey, i did the following two searches:,

and found another user asking for the same in Backblaze B2 integration as as well as a feature request in where some one said that this should be implemented as an ownCloud app and how to do that.