Refactor App::update

[reflash]

See ./src/Client/App.fs

let update msg model =
    match msg, model.CurrentPage with
    | NounDeclensionMsg msg, NounDeclension m ->
    ...
    | _, _ ->
        model, Cmd.none

This should be refactored as it can cause errors when some combinations are not handled. Current problem is that some combinations don't make sense e.g. NounDeclensionMsg, AdjectiveDeclension