Deduplicate U2F authentication views for users and admin mode

What does this MR do?

In preparation for further changes in this area (!26692 (merged), see also this note) this MR consolidates the duplicate U2F authentication HAML views used in app/views/admin/sessions/_two_factor_u2f.html.haml and app/views/u2f/_authenticate.html.haml by parameterizing and reusing the latter one.

Additionally, I renamed the HTML ids in the view to be less U2F (currently u2f -> 2fa-token, other suggestions welcome) specific to be able to reuse the partial later for WebAuthn.

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