Extract commonalities from trial-promo-related helpers

• https://gitlab.com/gitlab-org/gitlab/-/blob/v14.2.3-ee/ee/app/helpers/trial_status_widget_helper.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/v14.2.3-ee/ee/app/helpers/paid_feature_callout_helper.rb

Relates to both of these experiment cleanups:

• Display Active Trial Status (&6365)
• Highlight Paid Features During Active Trial (&6012)

I’m not sure if we’d want to simply extract the common portions to a new helper that the other two helpers each reuse, if we’d want to make a presenter (or presenters) instead of using helpers, or if we’d prefer to make some specific class(es) in ee/lib/ to encapsulate the functionality.