Fix bad URL in contact link on 404 page

[jabrah]

Fixes https://github.com/eclipse-pass/main/issues/806

Update 404 page to handle missing contact link

Changes

• Get rid of the template string, which would report undefined values as string literal 'undefined'
  • Message when URL is undefined changed to "Please contact your administrator"
• Is it fine to get rid of the stand alone get in this context?
• Update 404 page template
  • Don't add link if contact URL is not defined
  • Small restructure