Show a SSO login page for SSO users that just has a login button

(imported from commit 0004bf8710491f4e2d967b4367b37d57833845ee)

static/styles/portico.css

@@ -864,7 +864,7 @@ a.bottom-signup-button {
     font-weight: 300;
 }
 
-.login-google, .register-google {
+.login-google, .register-google, .login-sso {
     width: 300px;
     margin: auto;
     text-align: center;

templates/zerver/login.html

@@ -25,6 +25,16 @@ autofocus('#id_username');
 
     <h3 class="login-page-header">You look familiar.</h3>
 
+{% if only_sso %}
+    {% comment %}SSO users don't have a password.{% endcomment %}
+
+    <div class="login-sso">
+      <a href="/accounts/login/sso" class="btn btn-large btn-primary">Sign in with SSO</a>
+    </div>
+
+{% else %}
+{% comment %}Non-SSO users.{% endcomment %}
+
 {% if form.errors or sso_unknown_email %}
 <div class="alert alert-error">
     {% if sso_unknown_email %}
@@ -86,6 +96,8 @@ autofocus('#id_username');
     </div>
     {% endif %}
 
+{% endif %}
+
 </div>
 <div class="footer-padder"></div>
 </div>

zerver/context_processors.py

@@ -14,6 +14,7 @@ def add_settings(request):
         'zulip_admin':   settings.ZULIP_ADMINISTRATOR,
         'password_auth_enabled': password_auth_enabled(),
         'login_url':     settings.HOME_NOT_LOGGED_IN,
+        'only_sso':     settings.ONLY_SSO,
     }
 
 def add_metrics(request):