fix: Support icon display in development environment

[LachyTech]

This commit adjusts the CSS for icon backgrounds to use relative paths. In the dev environment this allows the icon selection form to display newly added icons in the source directory, and should also work for a production environment in the same way.

It is important that the icon paths have a hosted URI so that the final Markdown output is able to render the icons. During development this can be locally hosted but the Markdown output will not be usable without changes.

This commit also limits the displayed size of icons to fit within the container allowing larger icons to be used.

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[danielcranney]

Closing this PR as issues have been addressed in other PRs.

[LachyTech]

Hi mate, does the other work include the ability to see new icons in the dev environment? That was the main point of this PR, fixing the icon size was a bonus.

I'm happy to rebase this PR to retain the remainder of it.