updates to hoc sign up form, announcements, and highlights

[tanyaparker]

• Sign up form is above highlights during pre-hoc and soon-hoc, and below it all other times. We want to increase sign ups just during these two modes. Otherwise, during the actual Hour of Code and not Hour of Code season we de-prioritize it.
• When it's not Hour of Code season, we hide both the sign up form and the announcements. We show a "sign up is closed" message.
• Cleared the /learn and /volunteer announcements which are no longer needed. Now there is only one announcement: Robotics. The Robotics announcement is not really actionable/not related to the Robotics Activities listing, so I've removed the hyperlink and styling on hover that we tried to apply to make it look more clickable.
• Updating wiki

pre-hoc and soon-hoc

actual-hoc

post-hoc and false

[caleybrock]

lgtm