Improve "User Login" status tracking

[NiloCK]

Currently, login status is tracked only with a string representation of the username. Should instead be a richer object such as:

interface User {
  loggedIn: boolean,
  // ctx here refers to CouchDB's userContext object
  ctx: {
    name: string,
    roles: string[]
  } | undefined
}

Main nav menu should respond to user status:

• Only logged in users should have a 'Study' item
• Only logged in users with 'admin' role should have an 'Edit' button

Proper Vuex methods should be used to alter the user state in the store...