Add board sidebar item wrapper

What does this MR do?

This component would wrap any sidebar item, with this API:

<!-- the `ref` is for `$refs.sidebarItem.collapse()` -->
<board-editable-item
  ref="sidebarItem"
  :title="__('Sidebar item title')"
  :loading="isLoading"
  :can-update="userCanUpdate"
  @open="editModeOn = true"
  @closed="editModeOn = false"
  @changed="editModeOn = $event"
>
  <template #collapsed>
    Selected value is {{ selectedValue }}
  </template>

  <!-- shown when editing -->
  <template>
    <custom-item-select v-model="selectedValue" />
  </template>
</board-editable-item>

Screenshots

Used with assignees
5Q1C8Np7Xm

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done