Main app refactoring

[JWCook]

Just untangling a bit of spaghetti (and hopefully not adding more).

• Refactor get_inat_metadata() into observation_to_metadata() and iNatDbClient.from_ids()
• Use API client object attribute on main app instance instead of module-level object
• Use session object attribute on main app instance for fetching images instead of module-level object
• Access globally used objects via main app instance instead of passing them individually to controllers