UI problems on mobile

[benubah]

There are some issues with the UI on mobile as pointed out by @hturner:

• "Proudly supported by R Consortium" overlaps with copyright notice
• Layout is really squished so that you get thin column of text and phot that does not fit (3rd screenshot shows rest of phot if you horizontally scroll, which you wouldn't want to do on mobile)
• Transparent navbar is difficult to read when it overlays text underneath
• Squished layout is particularly bad for tables!

[benubah]

Transparent navbar is difficult to read when it overlays text underneath
Fixed in 690d8d4

"Proudly supported by R Consortium" overlaps with copyright notice
Use two columns instead of three in the footer