static: reinstate fixed navbar via flex

[haraldschilly]

Description

Testing Steps

I tested this in chrome and FF

Relevant Issues

Checklist:

• No debugging console.log messages.
• All new code is actually used.
• Non-obvious code has some sort of comments.

Front end:

• Restart at least one project and check its ~/.smc/local_hub/local_hub.log
• Completely restart Webpack with ./w in /src
• Completely restart the hub by killing and restarting ./start_hub.py in /src/dev/project
• Screenshots if relevant.

[williamstein]

What did you change? It's a LOT of work to tell from this patch, because you moved all the relevant code to another file, then changed it there...

[williamstein]

Well, I don't understand this, but it seems harmless, and worse case I can revert.

[haraldschilly]

yes, I also cleaned this up a bit, makes it hard to follow. The key idea is to split the top part in two divs and then

body
  display: flex
  flex-direction: column
  height: 100vh
  overflow: hidden

div#content
  flex: 1 1 auto
  overflow: auto

nav#nav-main
  flex: 0 0 auto