add valid-modes

[Zaturrby]

Adds new valid-modes to allow the overlays to be accessed.

Things to think about:

1. Some overlays will not always be valid (like new-user for a recurring user)
2. Naming of the overlays, the names chosen now match the ones chosen in offcourse-web.

[yeehaa123]

@Zaturrby

1. agree, so we'll have to dynamically limit the set of current valid states based on the rest of the app state
2. I think we can get away with 4 modes: normal-mode (don't call it view-mode it's confusing, because of the naming collision with viewmodels, etc), edit-mode, auth-mode, and focus-mode. The variations (new-user, etc.) can either be derived from appstate (auth can only be in new-user state if there is no valid user in our db for instance) or stored in a local atom.

Can you refactor this accordingly? I already made an issue (#2) for this.

[Zaturrby]

@yeehaa123 Will do!