Update NCAR and Unidata logos to be in compliance with new NSF guidance

[clyne]

The landing page, and any other page that contains NCAR or Unidata logos or mentions NCAR or Unidata, will need to be updated per NSF's new branding guidance.

[dcamron]

We should consider these in presentations, hackathons, etc. moving forward as well.

Todos here:

• Update any NCAR and Unidata logos. I've attached Unidata's logos here and can provide a Drive link as well
• Any text references of "NCAR" and "Unidata" should be updated to "NSF NCAR" and "NSF Unidata"
• We may want to make our footer and NSF acknowledgments clearer, where we should explicitly reference the "U. S. National Science Foundation"

Unidata logos:

Example acknowledgment text:

NSF Unidata is one of the University Corporation for Atmospheric Research (UCAR)'s Community Programs (UCP), and is funded primarily by the U. S. National Science Foundation (Grant AGS-1901712).