multi: Add Simnet support.

[victorgcramos]

This diff adds the support for Simnet config. [Work in Progress ⏳ ]

Simnet Environment requirements:

• Dcrd
• Dcrwallet
• Dcrdata

Config changes:

• Politeiawww:

  # Get your pubkey paywall from dcrwallet
  $ dcrctl --simnet --wallet --rpcuser=<username> --rpcpass=<password> getmasterpubkey <account>

  Paste the paywallxpub inside ~/.politeiawww/politeiawww.conf:

  ; ...
  ; Paywall configuration: xpub is used to derive paywall addresses
  ; and the paywall amount is specified in atoms.
  paywallxpub=<pubkey from dcrwallet simnet>
  ; ...