Refine UI of integration form

What does this MR do?

Part of #197297 (closed)

Split from !33839 (closed) to reduce scope and number of changes. Please review the commits list (and description) for detailed changes.

Note: The strange-looking UI with Active / Triggers will be fixed separately in !34697 (merged). It only shows up for Service templates (which is being deprecated) and Instance-level integrations (which is unreleased).

Screenshots

Section	Before	After	Changes
Project-level integrations			Enter new Token help text is rendered properly
Instance-level integrations (not released yet)			Width of content is limited. Sidebar is highlighted with the right section
Service templates			Width of content is limited

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