Consistent sizing in the frontend

[Iain-S]

We use a mix of %, px, vh, etc. for sizing on the frontend. We should use % in all cases to be consistent.

For why relative units are preferable, see e.g. https://gist.github.com/basham/2175a16ab7c60ce8e001

Changes will definitely need to be made to https://github.com/alan-turing-institute/rctab-api/blob/main/rctab/static/css/styles.css and, possibly, to html templates in the templates/ directory.