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/__tests__/__snapshots__/confirmation_popover.test.js.snap

@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`ConfirmationPopover matches snapshot 1`] = `<v-popover placement="top-start"><template></template> <button type="button" class="tooltip-target">Do something dangerous</button></v-popover>`;