Extract mixin to create a grid layout

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.

.stylelintrc.yml

@@ -83,6 +83,7 @@ rules:
   "@stylistic/indentation":
     - 2
     - ignore:
+      - param
       - value
   "@stylistic/media-feature-parentheses-space-inside": never
   "@stylistic/no-eol-whitespace": true