Make work item quick actions text consistent

What does this MR do and why?

Make work item quick actions text consistent:

• Don't say "child" or "children" on their own:
  • https://docs.gitlab.com/development/documentation/styleguide/word_list/#child
  • https://docs.gitlab.com/development/documentation/styleguide/word_list/#parent
• Add consistent punctuation to success messages.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

Make sure new look for issues is on:

• In an issue that has a parent epic assigned, type /parent or /child.
• In an epic, type /parent or /child.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.