Improve template of conflicts app

What does this MR do?

This moves (almost) all buttons to GitLab UI and uses the correct styling: confirm over success, selected over 'active'.

We are also moving the HTML around a little in order to align with our button guidelines (primary button should be next to cancel button) and keep the DOM simpler, removing redundant <div> and <template> elements. Also removed a few "useless" classes in favor of our utility classes.

Unfortunately I wasn't able to make the GitLab UI Form elements work properly with the textarea.

Screenshots (strongly suggested)

Before	After
conflicts-app-before	conflicts-app-after

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Related to #321090 (closed)