build: improve code-splitting and chunks cache

[tiagoalvesdulce]

Depends on https://github.com/decred/pi-ui/pull/466

• Lazy load routes (this separate routes in different chunks)
• Split pi-ui from vendors chunk (since pi-ui is a UI lib and can change frequently so we cache it separately)
• Cache vendors and pi-ui
• Gzip

Bundle size before (1.5Mb):

Bundle size after (482Kb gziped):

[victorgcramos]

Nice!