This stylelint-scss rule is useful because we were inconsistent when
using calc(); sometimes we added interpolation to Sass variables, and
sometimes we didn't. The reason why we originally added interpolation
was that it was necessary until we migrated to Dart Sass in commit
d54971e53. Since then, we can omit the interpolation, which is also what
the Sass documentation recommends [1].
[1] https://sass-lang.com/documentation/values/calculations/
Note that adding the labels broke the layout because the button was no
longer aligned with the fields, so we're now using a flex layout.
Since we're using labels, we no longer need a placeholder (which wasn't
very informative, by the way) in the text field.
The `clear` class isn't needed since commit c9f31b8e1, when we moved
this button above the regular search fields.
We're also moving the `float` property to the CSS file.
