This commit introduces an icon to all links in the sidebar. If an icon has not been configured, we will fall back to a generic "link" icon. As part of this commit, we also standardised the size of each prefix to 20px by 20px and set a fix margin. This is to allow sufficient space for text prefixes and image prefixes to be displayed.
Tests have been intentionally left out for now as I don't feel like asserting for the icons will bring much value at this point. Time shall prove me wrong.
Co-authored-by: awesomerobot <kris.aubuchon@discourse.org>