settings: Add confirm_dialog module.

This module makes it really easy to create are-you-sure
dialogs for dangerous operations.

Basically it's one function with five parameters.  You
give three chunks of HTML, a callback function, and
a parent container.

The first use of this will be in settings_user_groups,
coming up in a couple commits.

static/js/confirm_dialog.js

@@ -0,0 +1,86 @@
+var confirm_dialog = (function () {
+
+var exports = {};
+
+/*
+    Look for confirm_dialog in settings_user_groups
+    to see an example of how to use this widget.  It's
+    pretty simple to use!
+
+    Some things to note:
+
+        1) We create DOM on the fly, and we remove
+           the DOM once it's closed.
+
+        2) We attach the DOM for the modal to conf.parent,
+           and this temporary DOM location will influence
+           how styles work.
+
+        3) The cancel button is driven by bootstrap.js.
+
+        4) For settings, we have a click handler in settings.js
+           that will close the dialog via overlays.close_active_modal.
+
+        5) We assume that since this is a modal, you will
+           only ever have one confirm dialog active at any
+           time.
+
+*/
+
+exports.launch = function (conf) {
+    var html = templates.render("confirm_dialog");
+    var confirm_dialog = $(html);
+
+    var conf_fields = [
+        // The next three fields should be safe HTML. If callers
+        // interpolate user data into strings, they should use
+        // templates.
+        'html_heading',
+        'html_body',
+        'html_yes_button',
+        'on_click',
+        'parent',
+    ];
+
+    _.each(conf_fields, function (f) {
+        if (!conf[f]) {
+            blueslip.error('programmer omitted ' + f);
+        }
+    });
+
+    conf.parent.append(confirm_dialog);
+
+    // Close any existing modals--on settings screens you can
+    // have multiple buttons that need confirmation.
+    if (overlays.is_modal_open()) {
+        overlays.close_modal('confirm_dialog_modal');
+    }
+
+    confirm_dialog.find('.confirm_dialog_heading').html(conf.html_heading);
+    confirm_dialog.find('.confirm_dialog_body').html(conf.html_body);
+
+    var yes_button = confirm_dialog.find('.confirm_dialog_yes_button');
+
+    yes_button.html(conf.html_yes_button);
+
+    // Set up handlers.
+    yes_button.on('click', function () {
+        overlays.close_modal('confirm_dialog_modal');
+        conf.on_click();
+    });
+
+    confirm_dialog.on('hide', function () {
+        confirm_dialog.remove();
+    });
+
+    // Open the modal
+    overlays.open_modal('confirm_dialog_modal');
+};
+
+return exports;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = confirm_dialog;
+}
+window.confirm_dialog = confirm_dialog;