create stream: Don't allow creating stream with zero subscribers.

Restrict users(even realm admins) from creating stream with zero
subscribers only in UI.
In backend, if subscribers are zero, we automatically subscribe
current user to stream.

static/js/stream_create.js

@@ -16,6 +16,22 @@ exports.get_name = function () {
     return created_stream;
 };
 
+var stream_subscription_error = (function () {
+    var self = {};
+
+    self.report_no_subs_to_stream = function () {
+        $("#stream_subscription_error").text(i18n.t("You cannot create a stream with no subscribers!"));
+        $("#stream_subscription_error").show();
+    };
+
+    self.clear_errors = function () {
+        $("#stream_subscription_error").hide();
+    };
+
+    return self;
+
+}());
+
 var stream_name_error = (function () {
     var self = {};
 
@@ -224,6 +240,7 @@ exports.show_new_stream_modal = function () {
 
     stream_name_error.clear_errors();
     $(".stream_create_info").hide();
+    stream_subscription_error.clear_errors();
 
     $("#stream-checkboxes label.checkbox").on('change', function (e) {
         var elem = $(this);
@@ -326,6 +343,11 @@ $(function () {
         }
 
         var principals = get_principals();
+        if (principals.length === 0) {
+            stream_subscription_error.report_no_subs_to_stream();
+            return;
+        }
+
         if (principals.length >= 50) {
             var invites_warning_modal = templates.render('subscription_invites_warning_modal',
                                                          {stream_name: stream_name,

static/styles/subscriptions.css

@@ -241,7 +241,7 @@ form#add_new_subscription {
     width: calc(100% - 15px);
 }
 
-#stream_name_error {
+.stream_creation_error {
     display: none;
     margin-left: 2px;
     color: hsl(0, 100%, 50%);

static/templates/subscription_creation_form.handlebars

@@ -10,7 +10,7 @@
                 </div>
                 <input type="text" name="stream_name" id="create_stream_name"
                      placeholder="{{t 'Stream name' }}" value="" autocomplete="off" />
-                <div id="stream_name_error"></div>
+                <div id="stream_name_error" class="stream_creation_error"></div>
             </section>
             <section class="block">
                 <div class="stream-title">
@@ -60,6 +60,7 @@
                 <label class="stream-title" for="people_to_add">
                     {{t "People to add" }}
                 </label>
+                <div id="stream_subscription_error" class="stream_creation_error"></div>
                 <div class="controls" id="people_to_add"></div>
             </section>
         </div>