Add Unit Settings

[avatarneil]

First pass at #93... Tried abstracting things into lots of helper functions, and every approach just ended up feeling over-engineered. The meat and potatoes of this PR:

• Introduce a Settings page which allows a user to pick metric or imperial unit preference
• Introduce a little Unit component to display whatever the local unit should be
• Use the convert-units library for conversions

One thing I'd like to do before merging is get it so the settings page somehow indicates what your current setting is.

[avatarneil]

Apologies for all of the reformatting... Though the prettier config from #105 didn't come along with this PR, me setting my vscode to auto-format for vanilla js files did 😅

[kelvinfan001]

Haha, no worries. Though I think merging #105 first then rebasing this on top of it would make the git diff a bit easier to read.