Fix issues with CUGOS website on Mobile

[mizmay]

... currently has issues, exacerbated slightly but not created by #423. These are reproducible on desktop by reducing the viewport size on your browser to mobile screen width:

1. Drop-down menu is huge
2. Hover doesn't differentiate between top-level Fling and specific dates
3. Menu is transparent on some pages

[njhenry]

The transparent navbar hack was fun while it lasted, but I think its time has come—see also the styling of the navbar dropdown on fling pages:

[njhenry]

2 is now resolved in #423