Notifications page should be unified and simply use `UserState` for user specific functionality

[ynx0]

Currently, there are 3 separate routes per-user for notifications. This adds unnecessary complexity. Instead, the 3 pages should become 1 page and 1 route, where user specific rendering should be done using rx.cond and the UserState.role as appropriate.

cc @jalsayid

(Original comment #22 (comment))