Add export functionality to Sensors page

- Create TableToolbarExport component to be used as a slot
  in TableToolbar
- Allows selected table items to be exported

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: I929347e046af8a5d5188e4c4fd9fc874e067cce5
diff --git a/src/components/Global/TableToolbar.vue b/src/components/Global/TableToolbar.vue
index 041a9c1..331f007 100644
--- a/src/components/Global/TableToolbar.vue
+++ b/src/components/Global/TableToolbar.vue
@@ -15,6 +15,7 @@
           >
             {{ action.label }}
           </b-button>
+          <slot name="export"></slot>
           <b-button
             variant="primary"
             class="d-block"
@@ -38,7 +39,7 @@
     },
     actions: {
       type: Array,
-      required: true,
+      default: () => [],
       validator: prop => {
         return prop.every(action => {
           return (