Add support for multi currencies per Brand

[Kaian]

This PR allows configuring the currency symbol for each:

• Brand (default currency for all its items)
• Client
• Carrier
• Destination rate
• Rating plan

Important notes:

• Rating plans should only group destination rates using its currency.

• Clients and carriers should only use rating plans using its currency.

  • Balance movements will be done using their currency.
  • Invoices will use client's currency.
  • Invoices' fixed costs will use client's currency.

• Some backend checks avoid some of previous misconfigurations, but not all of them: use this feature carefully.

• There is no currency conversion involved.

  • Call cost will be calculated in carrier's currency.
  • Call price will be calculated in client's currency.

• LCR routes involving carriers with different currencies are not supported.

This PR fixes #636