Rename <firebase-storage-multiupload> to <firebase-storage-upload>

[FluorescentHallucinogen]

What about renaming <firebase-storage-multiupload> to just <firebase-storage-upload>?

[FluorescentHallucinogen]

@tjmonsi @mbleigh PTAL.

[mbleigh]

I'd like to leave the door open for a simpler element that is dedicated to uploading a single file. That being said, I'm trying to open this up more to the community since my time to maintain is super-limited, so if @tjmonsi wants to rename I wouldn't object (though since it's been released, this would be a breaking change).

[tjmonsi]

@mbleigh @FluorescentHallucinogen yeah it would be a breaking change. I think I can do a "similar" file that is extended from firebase-storage-multiupload but that would just be for Polymer 2.0 and above.

@mbleigh my doors are open to help maintain if you still need help :)