Move EE differences for `app/assets/javascripts/sidebar/components/todo_toggle/todo.vue`

The file app/assets/javascripts/sidebar/components/todo_toggle/todo.vue has differences between CE and EE.

Differences:

diff --git a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/assets/javascripts/sidebar/components/todo_toggle/todo.vue b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/assets/javascripts/sidebar/components/todo_toggle/todo.vue
index 706e6ca19c3..57125c78cf6 100644
--- a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/assets/javascripts/sidebar/components/todo_toggle/todo.vue
+++ b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/assets/javascripts/sidebar/components/todo_toggle/todo.vue
@@ -50,6 +50,9 @@ export default {
     buttonLabel() {
       return this.isTodo ? MARK_TEXT : TODO_TEXT;
     },
+    buttonTooltip() {
+      return !this.collapsed ? undefined : this.buttonLabel;
+    },
     collapsedButtonIconClasses() {
       return this.isTodo ? 'todo-undone' : '';
     },
@@ -69,7 +72,7 @@ export default {
   <button
     v-tooltip
     :class="buttonClasses"
-    :title="buttonLabel"
+    :title="buttonTooltip"
     :aria-label="buttonLabel"
     :data-issuable-id="issuableId"
     :data-issuable-type="issuableType"

What needs to be done:

1. Create a mixin for buttonTooltip computed property in the EE folder.
2. Create a CE mixin counterpart, with the same name.
3. In CE the buttonTooltip should return buttonLabel content.
4. Use ee_else_ce to import the mixin into this file.
5. Guarantee that specs exist or are added

Full plan.