Summary:
Changelog: Updated Network plugin to Sandy UI, including several UI improvements
Converted UI to Sandy, and some minor code cleanups
Moved all mock related logic to its own dir
Fixes https://github.com/facebook/flipper/issues/2267
Reviewed By: passy
Differential Revision: D27966606
fbshipit-source-id: a64e20276d7f0966ce7a95b22557762a32c184cd
