add Enable read-only to Settings page

app/assets/javascripts/admin/controllers/admin-site-settings.js.es6

@@ -1,4 +1,5 @@
 import debounce from 'discourse/lib/debounce';
+import { ajax } from 'discourse/lib/ajax';
 
 export default Ember.Controller.extend({
   filter: null,
@@ -70,7 +71,36 @@ export default Ember.Controller.extend({
 
     toggleMenu() {
       $('.admin-detail').toggleClass('mobile-closed mobile-open');
+    },
+
+    toggleReadOnlyMode() {
+      var self = this;
+      if (!this.site.get("isReadOnly")) {
+        bootbox.confirm(
+          I18n.t("admin.backups.read_only.enable.confirm"),
+          I18n.t("no_value"),
+          I18n.t("yes_value"),
+          function(confirmed) {
+            if (confirmed) {
+              Discourse.User.currentProp("hideReadOnlyAlert", true);
+              self._toggleReadOnlyMode(true);
             }
           }
+        );
+      } else {
+        this._toggleReadOnlyMode(false);
+      }
+    }
+  },
+
+  _toggleReadOnlyMode(enable) {
+    var site = this.site;
+    ajax("/admin/backups/readonly", {
+      type: "PUT",
+      data: { enable: enable }
+    }).then(() => {
+      site.set("isReadOnly", enable);
+    });
+  }
 
 });

app/assets/javascripts/admin/templates/site-settings.hbs

@@ -1,5 +1,10 @@
 <div class='admin-controls'>
   <div class='search controls'>
+    {{#if site.isReadOnly}}
+      {{d-button icon="eye" action="toggleReadOnlyMode" disabled=status.isOperationRunning title="admin.backups.read_only.disable.title" label="admin.backups.read_only.disable.label"}}
+    {{else}}
+      {{d-button icon="eye" action="toggleReadOnlyMode" disabled=status.isOperationRunning title="admin.backups.read_only.enable.title" label="admin.backups.read_only.enable.label"}}
+    {{/if}}
     <label>
       {{input type="checkbox" checked=onlyOverridden}}
       {{i18n 'admin.site_settings.show_overriden'}}