Slightly improve header accessibility.

Partially addresses: https://meta.discourse.org/t/links-without-text-make-blind-user-accessibility-worse/13538.
Remove extra anon glyphs, move sign in button.

See: https://meta.discourse.org/t/add-sign-in-to-anonymous-header/15335
Use helper to insert sr-only text for icons.
Merge pull request #2301 from vikhyat/accessibility

Slightly improve header accessibility