Change trials_link_url name to callouts_trials_link_url

What does this MR do and why?

Closes: #351319 (closed)

glm_content should be top-right-dropdown in user dropdown but we got gold-callout. This problem is because of the trials_link_url has a duplicate name method in the app/helpers/users_helper.rb.

So rename it to callouts_trials_link_url.

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Related to #351319 (closed)