Session grid works poorly on small screens

[stefanbethke]

Describe the bug When viewing the session grid on a small screen (iPhone 7 Plus or smaller), it's hard to understand.

To Reproduce Use http://lab.maltewassermann.com/viewport-resizer/ or similar to view site with a small viewport.

Expected behavior The session grid should be readable even on small screens.

Screenshots

[jandd]

Looks better with #150