Adjust display settings for MR widget source branch removal components

What does this MR do?

1. In the MR Widget, when user can't remove source branch, we hide the checkbox
2. In the MR Create/Edit View, when user can't remove source branch, we hide the checkbox
3. In the MR Widget, when the user can't remove the source branch and the source branch will be removed, show a message in the MR Widget that says "Removes source branch".
4. Update existing specs and add new ones for the "Removes source branch" status message
5. Adds some logic to the spec helper method mountComponent to support passing plain Vue config objects

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #33264 (moved)