Grey out Learn Day Sign Up buttons and disable all sign up

[acchiang]

A couple of tasks to close registration (this could be a good toggle to add to the CMS in the future):

• Grey out the Sign Up hero button
• Remove link to Sign Up on hero button
• Grey out the Learn Day Sign Up button
• Remove link to Sign Up on hero button in Firebase
• Update description from "Learn Day registration is now open! Spots are limited" TO "Learn Day registration is closed! Join the mailing list below for future nwPlus events."

Verify:

• Hero button is unclickable in normal browser mode AND incognito mode
• Description is updated and looks ok
• There are no results when you command+F for organize.mlh.io in dev tools (Right click > Inspect element)

[acchiang]

Hero links were last changed here by @luceliu : https://github.com/nwplus/LHD2020/pull/37

[acchiang]

Closing this issue since we have pushed out modularization (is that a word?) for these -- and @Ciuffi updated the website as such:

• Sign Up buttons can remain not greyed out, but we will link them to the WAITLIST instead of the registration link: https://forms.gle/Lzh287Yh8KZNHVYw5
• The buttons can still say “Sign Up”
• Description on hero section below hero buttons now say “Learn Day registration is closed! Sign up to our waitlist.”