SideNav icons have inconsistent sizes

[ggascoigne]

svgs are 50px, but font icons (such as many in the style guide) get no default size.

Also, both TOC and TFD appear to be using 40px square icons, so it seems likely that the 50px default is out of date.

[andrewleyva]

16, 24, 32, 48 etc. are my vote for sizes. This has been the route we've been going in new designs.
Perhaps we should add guidance in the Octagon Design System for this; I'll defer to UX.
@ykohux, @johnnylevenson