Move variables & logic from view into helpers

What does this MR do?

This MR cleans up the ee/app/views/trials/select.html.haml view a bit by moving a couple of local variables into helper methods (Fat Helpers, Skinny Views). This also makes the logic of those variables easier to test.

Note: even more cleanup of this view is being done in !41020 (merged).

As this is a completely developer-facing change, no changelog entry or documentation is necessary.

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