[JENKINS-69653] Un-inlining dialog.jelly

[Pldi23]

Hello,

Hacktoberfesting to solve JENKINS-69653.

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

Manually test to ensure 'Cancel' and 'Add' buttons works as before:

https://user-images.githubusercontent.com/47667812/196411047-fda17d60-dcd1-44a1-8dc6-c9e02a001bfe.mov

[Pldi23]

@Wadeck @daniel-beck (can't add reviewers, so mentioning you here)