Removed some un needed flash messages raised as bugs.

In the process found a couple of edge cases of incorrect
use of invitation links by other users which are now
handled.

app/main/views/sign_in.py

@@ -4,7 +4,8 @@ from flask import (
     url_for,
     session,
     flash,
-    request
+    request,
+    abort
 )
 
 from flask.ext.login import (
@@ -13,26 +14,40 @@ from flask.ext.login import (
     confirm_login
 )
 
+from app import (
+    user_api_client,
+    invite_api_client,
+    service_api_client
+)
+
+
 from app.main import main
-
-from app import (user_api_client, service_api_client)
-
-
 from app.main.forms import LoginForm
 
 
 @main.route('/sign-in', methods=(['GET', 'POST']))
 def sign_in():
+
     if current_user and current_user.is_authenticated():
         return redirect(url_for('main.choose_service'))
 
     form = LoginForm()
     if form.validate_on_submit():
+
         user = user_api_client.get_user_by_email_or_none(form.email_address.data)
         user = _get_and_verify_user(user, form.password.data)
         if user and user.state == 'pending':
             flash("You haven't verified your email or mobile number yet.")
             return redirect(url_for('main.sign_in'))
+
+        if user and session.get('invited_user'):
+            invited_user = session.get('invited_user')
+            if user.email_address != invited_user['email_address']:
+                flash("You can't accept an invite for another person.")
+                session.pop('invited_user', None)
+                abort(403)
+            else:
+                invite_api_client.accept_invite(invited_user['service'], invited_user['id'])
         if user:
             # Remember me login
             if not login_fresh() and \
@@ -56,11 +71,6 @@ def sign_in():
         # Vague error message for login in case of user not known, locked, inactive or password not verified
         flash('Username or password is incorrect')
 
-    invited_user = session.get('invited_user')
-    if invited_user:
-        message = 'You already have an account with GOV.UK Notify. Sign in to your account to accept this invitation.'
-        flash(message, 'default')
-
     return render_template('views/signin.html', form=form)