Prevent sass deprecation warnings

[jcushman]

As a bit of housekeeping, this prevents deprecation notices during sass compile from using slash as a division operator and using minus-sign-variable without parens.

As suggested by sass, I did

npm install -g sass-migrator
sass-migrator division static/css/*.scss
sass-migrator strict-unary static/css/*.scss

and then went and cleaned up the changes.

Fun fact for division: these are mostly in an early section of the css that must have an interesting story behind it. There's a bunch of stuff like .col-sm-10by3 { width: 100% * math.div(5, 18); // 10/3 width } that looks like it was copied from bootstrap, modified, and in some cases not currently used. I didn't go down the rabbit hole of figuring out what should be imported from bootstrap or deleted.

Fun fact for unary negation: I believe the rules strict-unary updated were in fact not operating as intended, which of course is why they started printing warnings about them. There's stuff in here like margin: 0 -$grid $double -$grid; which apparently would have been parsed as margin: (0-$grid) ($double-$grid), which I'm sure can't be what the author meant. I arbitrarily chose to have these do what they read like rather than how browsers have been interpreting them, by updating to margin: 0 (-$grid) $double (-$grid);, so at least the authorial intent will be visible if anyone ever goes back to revise. I looked at the affected elements (the single-link menu dropdown, and the link browser item-containers) and they look fine to me.

[rebeccacremona]

😁 This is such a good one.... we'll never know. Thanks for doing this!