In order to make API requests, the client needs to
fetch a JWT token and then configure 'angular-jwt'
to provide it on subsequent HTTP requests to
API endpoints.

This fairly simple task was complicated by
the inclusion of an emulation of the deprecated
Mozilla IdentityManager API - see https://developer.mozilla.org/en-US/docs/Web/API/IdentityManager

This commit replaces the identity module with
a much simpler implementation that only does
what we actually need at present:

 1. Enable the 'angular-jwt' interceptor which
    adds 'Authorization: Bearer <Token>' headers
    to API HTTP requests.

 2. Provide the JWT interceptor with a function
    which fetches JWT tokens and caches them.

The new implementation fixes two bugs in the previous
implementation:

 1. Cached API tokens were not invalidated properly when
    signing out (#3083).

    (In the old code, 'authPromise' was set to a rejected promise
     after signing out, but 'checkAuthentication()' checked for
     'authPromise' being _null_ when deciding whether to retrieve
     a new token. Consequently API requests made immediately
     after signing in could end up being unauthenticated).

 2. The value of $scope.auth.username and session.state.userid
    could get out of sync (#2924).

    In the new implementation, $scope.auth.username is always
    updated whenever the USER_CHANGED event is emitted and that
    event is always emitted when session.state.userid changes.

Fixes #3083
Fixes #2924