Show a more useful message if you get signed out

> Users that allow their session to expire, or access a bookmarked link
> are told they need to "Sign in to access this page" - we should
> explain that it's because they've been away a while, so that they
> understand why they're being asked to log in again.

– https://www.pivotaltracker.com/story/show/140016919

The message we were showing before (Please log in to access this page is
the default message from Flask Login).

In order to stop this flash message from appearing, we need to override
the default handler for when a user is unauthorised. We’re overriding it
with the same behaviour, minus the flash message.

If you navigate deliberately to the sign in page it’s unchanged.

Content is Sheryll-approved.

app/main/views/sign_in.py

@@ -16,6 +16,7 @@ from flask_login import (
 )
 
 from app import (
+    login_manager,
     user_api_client,
     service_api_client,
     invite_api_client
@@ -75,7 +76,14 @@ def sign_in():
             ).format(password_reset=url_for('.forgot_password'))
         ))
 
-    return render_template('views/signin.html', form=form)
+    return render_template('views/signin.html', form=form, again=bool(request.args.get('next')))
+
+
+@login_manager.unauthorized_handler
+def sign_in_again():
+    return redirect(
+        url_for('main.sign_in', next=request.path)
+    )
 
 
 def _get_and_verify_user(user, password):

app/templates/views/signin.html

@@ -10,9 +10,19 @@
 
 <div class="grid-row">
   <div class="column-two-thirds">
-    <h1 class="heading-large">Sign in</h1>
 
-    <p>If you do not have an account, you can <a href="register">create one now</a>.</p>
+    {% if again %}
+      <h1 class="heading-large">You need to sign in again</h1>
+      <p>
+        We sign you out if you haven’t used Notify for a while.
+      </p>
+    {% else %}
+      <h1 class="heading-large">Sign in</h1>
+      <p>
+        If you do not have an account, you can
+        <a href="{{ url_for('.register') }}">create one now</a>.
+      </p>
+    {% endif %}
 
     <form method="post" autocomplete="nope">
       {{ textbox(form.email_address) }}