Extract integration trigger fields to Vue

What does this MR do?

This is one of a series of MRs to migrate the form to move the project integrations form from HAML to Vue. The first two !27778 (merged) and !30258 (merged) have already been merged.

This MR moves the TriggerFields section into a component:

• Behind a feature flag integration_form_refactor. This ff will NOT be enabled until we have a version with consistent UI.
• Pass data from backend via data attributes.
• Use GitLab UI for all elements.

Screenshots

Note: Yes the UI is inconsistent. No it will not be displayed to the user as-is.

Integration	Before (without ff enabled)	After (with ff enabled)
Slack notifications
Hangouts Chat

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team