Select CI template based on URL param

What does this MR do?

It adds the possibility to define a pre-selected CI template for the editor via a template URL param.

E.g. URL: http://localhost:3000/gitlab-org/gitlab-shell/-/new/master?commit_message=Add+.gitlab-ci.yml&file_name=.gitlab-ci.yml&template=Android

This is needed for gitlab-org/growth&61 (closed) where we want to list templates for the pipeline zero state page and link the user to the given template.

Screenshots (strongly suggested)

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

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• 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