Execute quick actions when creating MR from issue

What does this MR do?

In CE: not much. The MergeRequests::BuildService will never set a description if one isn't passed.

In EE: you can have a default MR template, so this ensures quick actions work when set in the default MR template, and you create an MR from an issue using the handy button.

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

I would hope this is covered well enough by specs that this doesn't have any nasty edge cases.

Why was this MR needed?

It's a bug in EE, but the fix should be in CE, for consistency.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4277.