Add coin base unit [draft]

[rowandh]

• Adds configurable baseunit
• We don't currently store any preferences locally, so I used localStorage. Electron seems to support this fine but we can change it if needed.

See 3091.

TODO:

• There's a couple of spots where the base unit still needs to be added

[dev0tion]

Nice work, ping me if you want a review. Does using localStorage keep the settings between sessions?

[rowandh]

It does appear to, yes.