Draft: re-organizing types

Overview

This WIP MR moves

• Structured types
• Serialization and de-serialization functions (i.e. JSON decoders)

for Glance images

into new modules:

• src/OpenStack/Image.elm
• src/OpenStack/ImageStatus.elm
• src/OpenStack/ImageVisibility.elm

Everything else is just changes to make the refactor work.

Related to (but not a fix for) #917.

How to Test

Screenshots