Update Firebase Storage directory structure

[paulb777]

See https://github.com/firebase/firebase-ios-sdk/blob/master/AddNewPod.md#directory-structure

[morganchen12]

Please update the Danger PR autolabeler as well: https://github.com/firebase/firebase-ios-sdk/blob/master/Dangerfile#L90

[ryanwilson]

This LGTM but one concern I have is a large amount of the files showing up as new instead of moved - only worry being it's not as easy to see a blame of each file.

[paulb777]

Thanks for the push. There was an extra copy of those files. Now the diff should be right.

[ryanwilson]

Much better diff, thank you!