made sidebar + header bar sticky

[kohrachel]

As said in title. Changed some ordering of components in appRoutes.tsx: moved sidebar outside of the <div> for the home page, and added some conditional padding in the home page's <div> to ensure the sidebar doesn't overlap the content.

Remaining TODOs:

• @kohrachel fix sidebar behavior on some mobile/tablet screens (add breakpoints for collapsing)
• fix home page behavior -- background color doesn't fill the whole screen (see the second preview with the narrower screen, the gray stops before the end of the page).
  • Note: setting height: min-content on the parent element solves this but broke the centering of the sidebar for me

Some previews below:

Full-screen (desktop)

Narrow-screen (desktop)

[yifeifang11]

yes this looks awesome thank you! maybe this can be next week's task (to make sure the background behind the sidebar is also grey) but this works perfectly for our midpoint symposium