Refactor gui cache

[jholdstock]

Remove static cfg values from GUI cache.

Theres no need for these values to be copied into the cache when templates could simply access the config struct directly.

This also changes the cache accessor so it returns a copy of the cache rather than a pointer, which removes a potential race.

Rename and move cache.

Cache code was previous in homepage.go, but its used in multiple places and not just on the homepage. Its enough code to go into its own dedicated cache.go.

[jholdstock]

Still seeing races from the cache:

==================
WARNING: DATA RACE
Write at 0x00c0001143e0 by goroutine 43:
  github.com/decred/vspd/webapi.Start.func3()
      /home/jholdstock/code/jholdstock/vspd/webapi/webapi.go:160 +0x1cb

Previous write at 0x00c0001143e0 by goroutine 42:
  github.com/decred/vspd/webapi.Start.func2()
      /home/jholdstock/code/jholdstock/vspd/webapi/webapi.go:133 +0x75

Goroutine 43 (running) created at:
  github.com/decred/vspd/webapi.Start()
      /home/jholdstock/code/jholdstock/vspd/webapi/webapi.go:151 +0x1144
  main.run()
      /home/jholdstock/code/jholdstock/vspd/vspd.go:100 +0xed7
  main.main()
      /home/jholdstock/code/jholdstock/vspd/vspd.go:35 +0x9e

Goroutine 42 (finished) created at:
  github.com/decred/vspd/webapi.Start()
      /home/jholdstock/code/jholdstock/vspd/webapi/webapi.go:132 +0x1071
  main.run()
      /home/jholdstock/code/jholdstock/vspd/vspd.go:100 +0xed7
  main.main()
      /home/jholdstock/code/jholdstock/vspd/vspd.go:35 +0x9e
==================

[jholdstock]

Fixed the race, ready to review