[DX] [CMI] Add a site-wide concept of storage state (like ctools)

It would be useful if we had a global concept of storage state, for all modules and systems to use consistently. For example, a view can be custom, default, or overridden. So can a layout. So can anything else that's exportable, like an image style!

Right now views uses constants for these things (VIEWS_STORAGE_OVERRIDE, VIEWS_STORAGE_NORMAL) but it would be good if we didn't need to use these constants for other unrelated things, like layouts or image styles. It would also be good if they were not these silly constants (integers) but were instead strings, like "custom", "default", or "overridden".