Investigate px/rem sizing units

[retrixe]

We should have a Spacer component in Heliodor, <br> carries semantic meanings that disrupt usability and accessibility

[retrixe]

Also, hijacking this issue, we should have consistent usage of rem and px, probably just unify on rem except for borders and such, look at the regex px[;)"]

[retrixe]

Good news: We don't really have much use of <br> outside settings, where I've eliminated it.

Also good news: I've spent a lot of time hacking on this in Heliodor to make components scale better with different font sizes, and now I've experimented with this further in Concinnity (the new settings panel makes mixed use of px and rem components where linear scaling with font sizes makes good sense i.e. most of the time, it only doesn't when providing gaps between aesthetic components like boxes)

Bad news: We may still want to investigate the usage of px[;)" ] and rem[;)" ] units further in other components. So far things seem good when I up the font size, but we could probably do better.