Fix back btn for Polymer (#4467)

src/panels/config/cloud/account/cloud-account.js

@@ -64,7 +64,7 @@ class CloudAccount extends EventsMixin(LocalizeMixin(PolymerElement)) {
         }
       </style>
       <hass-subpage
-        showBackButton="[[!isWide]]"
+        show-back-button="[[!isWide]]"
         header="[[localize('ui.panel.config.cloud.caption')]]"
       >
         <div class="content">

src/panels/config/cloud/login/cloud-login.js

@@ -73,7 +73,7 @@ class CloudLogin extends LocalizeMixin(
         }
       </style>
       <hass-subpage
-        showBackButton="[[!isWide]]"
+        show-back-button="[[!isWide]]"
         header="[[localize('ui.panel.config.cloud.caption')]]"
       >
         <div class="content">

src/panels/config/core/ha-config-core.js

@@ -35,7 +35,7 @@ class HaConfigCore extends LocalizeMixin(PolymerElement) {
 
       <hass-subpage
         header="[[localize('ui.panel.config.core.caption')]]"
-        showBackButton="[[!isWide]]"
+        show-back-button="[[!isWide]]"
       >
         <div class$="[[computeClasses(isWide)]]">
           <ha-config-section-core

src/panels/config/server_control/ha-config-server-control.js

@@ -35,7 +35,7 @@ class HaConfigServerControl extends LocalizeMixin(PolymerElement) {
 
       <hass-subpage
         header="[[localize('ui.panel.config.server_control.caption')]]"
-        showBackButton="[[!isWide]]"
+        show-back-button="[[!isWide]]"
       >
         <div class$="[[computeClasses(isWide)]]">
           <ha-config-section-server-control

src/panels/config/users/ha-config-user-picker.js

@@ -60,7 +60,7 @@ class HaUserPicker extends EventsMixin(
 
       <hass-subpage
         header="[[localize('ui.panel.config.users.picker.title')]]"
-        showBackButton="[[!isWide]]"
+        show-back-button="[[!isWide]]"
       >
         <ha-card>
           <template is="dom-repeat" items="[[users]]" as="user">