[Angular - NgRx - SCSS] Cleanup and better organize interface files

[lindakatcodes]

With the service refactors in #440, there will be a number of interfaces that are no longer needed. Also, there are interface files located next to services, and interfaces within the state files for each folder.

This ticket would be to better organize the interfaces, and clean up anything that's no longer needed.

State files should have an interface for the global state (what we use for the initial state and what we expect that page's state to look like) and any small helper interfaces that make sense.

Service interfaces should be the interfaces needed for the API interactions and any small helper interfaces that go along with that.

If we would end up with an interface that would be helpful for both places, prioritize those going into the service interface and being imported into the state interface file.

Added note: As much as possible, let's make any app-facing interfaces use camelCase for variable names.