Fix partners display below 400px wide

Revert changes in 4ff40bb243d60f92efb6e3d36c1151db90837f60 to restore
flex-grid around contributors link, so that the link remains visible at
smaller screen widths.

Remove heights, overflow-hidden, and line-heights from sponsor text that
prevent it from wrapping readably.

Fixes AdmitHub/Admit#150.