Rename "Local" settings in Diffusion to "Storage"

Summary: Ref T2783. In Diffusion -> Edit Repository, we currently have a section called "Local" with options about where the repository is stored. The current name is misleading in a cluster environment, where storage may not actually be local. Shortly, this will also have an option for cluster storage. Call this "Storage" instead.

Test Plan: Edited a repository and poked around.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11001