Adjust signup page items for more clarity

What does this MR do and why?

Adjust signup page items for more clarity

• move omniauth provider buttons to bottom of trial registration
• move the terms above the for submit on regular and trial registration
• streamlined registration haml templates/partials and moved them into devise/registrations if only used by registration
• see https://gitlab.com/gitlab-org/gitlab/-/issues/482929

Changelog: other EE: true

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Screenshots or screen recordings

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Page	Before	After
regular signup
trial registration with duo_enterprise_trials_registration enabled
trial registration with duo_enterprise_trials_registration disabled	n/a

How to set up and validate locally

1. Simulate SaaS and restart GDK
2. Sign out if necessary.
3. Navigation to the /-/trial_registrations/new and /users/sign_up
4. To see the differences with 60 day text, for duo, use the command Feature.enable(:duo_enterprise_trials_registration) in the rails console and visit the trial registration path again.

Related to #482929