Add JuliaCon Check in page

[logankilpatrick]

Per @IanButterworth on Slack:

How about if there’s a “check-in” page that guides the user as if they’ve just arrived at the event. Something friendly like:
welcome to juliacon, make sure to:

• Sign up to the event discord - track discussions will happen here, including live audio rooms
• Sign up for slack and/or Zulip - the places where general chat happens
  Etc.
  Make it a semi-formal check-in process