Removed rem size hack, closes #192

[raikasdev]

Removing the rem size hack as described in #192 and replacing it with a SASS function (from the article mentioned in the issue
).

The sizes might be wonky! The change from 1rem = 10px to rem function was made with VSCode replace using Regex, so errors are possible. No problems with building or stylelint, though.

Note: CSS variables must use the compile time syntax #{rem(16px)} for it to work. Using it straight in statements or in SCSS variables is fine.

Closes #192

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[ronilaukkarinen]

Hmm, I'm really not a fan of using a function or mixin for sizes that are everywhere... Rather than this could we just change the math here and let stylelint-rem-over-px to take care of the conversion? We could change the math and then, change/remove the root size for body and then just go through every font size and re-save the files?