[UI] use vector or Retina icons for Wi-Fi and cellular bars

[ZevEisenberg]

I could try making them in Sketch, if you don't mind having a Sketch file checked into the repo, @twostraws.

[pluddy]

I tried to address this in #55
let me know if those icons look off, I tried to just use some pdf exported SF Symbols

[davedelong]

Fixed in #55