Add vue component for confirmation popover

Have the confirmation popover in a separate Vue component fixes a bug in
IE that was causing the `form` element in the popover to be ignored.

Since `form`s cannot be nested, the `form` element in the popover was
being discarded by IE and the revoke/resend invitation buttons did
nothing.

Breaking the functionality into a Vue component moves the `form` into a
separate template. When the popover is displayed, the component is added
to the DOM at the end, so the `form` is properly not-nested.

js/components/confirmation_popover.js

@@ -0,0 +1,32 @@
+export default {
+  name: 'confirmation-popover',
+
+  props: {
+    action: String,
+    btn_text: String,
+    cancel_btn_text: String,
+    confirm_btn_text: String,
+    confirm_msg: String,
+    csrf_token: String
+  },
+
+  template: `
+    <v-popover placement='top-start'>
+      <template slot="popover">
+        <p>{{ confirm_msg }}</p>
+        <div class='action-group'>
+          <form method="POST" v-bind:action="action">
+            <input id="csrf_token" name="csrf_token" type="hidden" v-bind:value="csrf_token">
+            <button class='usa-button usa-button-primary' type='submit'>
+              {{ confirm_btn_text }}
+            </button>
+          </form>
+          <button class='usa-button usa-button-secondary' v-close-popover>
+            {{ cancel_btn_text }}
+          </button>
+        </div>
+      </template>
+      <button class="tooltip-target" type="button">{{ btn_text }}</button>
+    </v-popover>
+  `
+}