Add Terms intermediary page

What does this MR do?

• Add required_elements and visible methods to base
• Add E2E spec for testing Terms page
• Allow login flow to return yielded result
• Allow element_selector_css to return selector_css if passed is an element
• Consume required_elements from Page::Base from Validatable

Closes #212145 (closed)

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