Sticky submit buttons in work item epic modal

What does this MR do and why?

Resolves #497863 (closed)

• Sticky submit buttons in work item epic modal
• Make work item create modal scrollable (slightly less tall)

Changelog: changed

Before	After
non-modal is same

How to set up and validate locally

1. In rails console

   Feature.enable(:namespace_level_work_items)

2. Go to epics/new page and look at the buttons
3. In left sidebar, go to + button and New Epic
4. Look at those buttons
5. Change window width/height and check things don't look weird
6. Look at those buttons
7. Click on the icon to open the form in a new page
8. Look at those buttons

app/assets/javascripts/work_items/components/create_work_item.vue

@@ -730,7 +730,8 @@ export default {
           </aside>
           <div
             v-if="!isModal"
-            class="gl-justify-content-end gl-col-start-1 gl-flex gl-gap-3 gl-py-3"
+            class="gl-col-start-1 gl-flex gl-gap-3 gl-py-3"
+            data-testid="form-buttons"
           >
             <gl-button
               variant="confirm"
@@ -749,8 +750,12 @@ export default {
         <!-- bg-overlap to match modal bg -->
         <div
           v-if="isModal"
-          class="gl-justify-content-end gl-sticky -gl-bottom-3 gl-bottom-0 gl-z-1 gl-col-start-1 -gl-ml-5 gl-flex gl-flex-row-reverse gl-gap-3 gl-bg-overlap gl-py-3"
+          class="gl-sticky -gl-bottom-3 gl-bottom-0 gl-z-1 -gl-ml-5 gl-flex gl-justify-end gl-gap-3 gl-bg-overlap gl-py-3"
+          data-testid="form-buttons"
         >
+          <gl-button type="button" data-testid="cancel-button" @click="handleCancelClick">
+            {{ __('Cancel') }}
+          </gl-button>
           <gl-button
             variant="confirm"
             :loading="loading"
@@ -759,9 +764,6 @@ export default {
           >
             {{ createWorkItemText }}
           </gl-button>
-          <gl-button type="button" data-testid="cancel-button" @click="handleCancelClick">
-            {{ __('Cancel') }}
-          </gl-button>
         </div>
       </div>
     </template>