Pajamas migrations

What does this MR do and why?

Migrates buttons in:

• app/views/layouts/header/_default.html.haml
• app/views/layouts/header/_sign_in_register_button.html.haml

Screenshots or screen recordings

	Before	After
Sign in / Register
Sign up now
Stop impersonation

How to set up and validate locally

For login buttons:

• Sign out
• Sign up now should be in top right corner
• Size browser down and open menu to see Sign in / register

For impersonation:

• Go to admin > users
• Select any user
• Click Impersonate at top right
• Stop impersonation will be in nav at top right

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #384422 (closed) and #384405 (closed)