Make user API client return JSON, not a model

The data flow of other bits of our application looks like this:
```
                         API (returns JSON)
                                  ⬇
          API client (returns a built in type, usually `dict`)
                                  ⬇
          Model (returns an instance, eg of type `Service`)
                                  ⬇
                         View (returns HTML)
```
The user API client was architected weirdly, in that it returned a model
directly, like this:

```
                         API (returns JSON)
                                  ⬇
    API client (returns a model, of type `User`, `InvitedUser`, etc)
                                  ⬇
                         View (returns HTML)
```

This mixing of different layers of the application is bad because it
makes it hard to write model code that doesn’t have circular
dependencies. As our application gets more complicated we will be
relying more on models to manage this complexity, so we should make it
easy, not hard to write them.

It also means that most of our mocking was of the User model, not just
the underlying JSON. So it would have been easy to introduce subtle bugs
to the user model, because it wasn’t being comprehensively tested. A lot
of the changed lines of code in this commit mean changing the tests to
mock only the JSON, which means that the model layer gets implicitly
tested.

For those reasons this commit changes the user API client to return
JSON, not an instance of `User` or other models.

app/main/views/two_factor.py

@@ -8,13 +8,14 @@ from flask import (
     session,
     url_for,
 )
-from flask_login import current_user, login_user
+from flask_login import current_user
 from itsdangerous import SignatureExpired
 from notifications_utils.url_safe_token import check_token
 
 from app import user_api_client
 from app.main import main
 from app.main.forms import TwoFactorForm
+from app.models.user import User
 from app.utils import redirect_to_sign_in
 
 
@@ -30,7 +31,7 @@ def two_factor_email_sent():
 @main.route('/email-auth/<token>', methods=['GET'])
 def two_factor_email(token):
     if current_user.is_authenticated:
-        return redirect_when_logged_in(current_user.id)
+        return redirect_when_logged_in(platform_admin=current_user.platform_admin)
 
     # checks url is valid, and hasn't timed out
     try:
@@ -79,27 +80,27 @@ def _is_safe_redirect_url(target):
 
 def log_in_user(user_id):
     try:
-        user = user_api_client.get_user(user_id)
+        user = User.from_id(user_id)
         # the user will have a new current_session_id set by the API - store it in the cookie for future requests
         session['current_session_id'] = user.current_session_id
         # Check if coming from new password page
         if 'password' in session.get('user_details', {}):
-            user = user_api_client.update_password(user.id, password=session['user_details']['password'])
-        activated_user = user_api_client.activate_user(user)
-        login_user(activated_user)
+            user.update_password(session['user_details']['password'])
+        user.activate()
+        user.login()
     finally:
         # get rid of anything in the session that we don't expect to have been set during register/sign in flow
         session.pop("user_details", None)
         session.pop("file_uploads", None)
 
-    return redirect_when_logged_in(user_id)
+    return redirect_when_logged_in(platform_admin=user.platform_admin)
 
 
-def redirect_when_logged_in(user_id):
+def redirect_when_logged_in(platform_admin):
     next_url = request.args.get('next')
     if next_url and _is_safe_redirect_url(next_url):
         return redirect(next_url)
-    if current_user.platform_admin:
+    if platform_admin:
         return redirect(url_for('main.platform_admin'))
 
     return redirect(url_for('main.show_accounts_or_dashboard'))