We were using similar code and three places. And all of them used
`15rem`, which looked a lot like a magic number. So we're making it the
default value of a mixin, which means replacing it with a less arbitrary
value will be easier.
Note that, for the column gap, we're now using the standard
grid-column-gutter we use in most places. We were using `$line-height`
in a couple of places only because writing it is less verbose.
Since we're now, for the first time, using a very long mixin definition
that we need to split in several lines, we're adding the `param`
exception to the indentation rule. As far as I know, there's no way to
define a rule in Stylelint that requires parameters in multiple lines to
be aligned with the first parameter, which is what we define in Rubocop.
29622dcb91