Add weighted averages report to web ver

[ZIMkaRU]

This PR adds the Weighted Averages report to the web version. The main part of logic moves from the bfx-reports-framework and the behavior is the same as in the previous PR bitfinexcom/bfx-reports-framework#246

---

Breaking changes

Response example in case trades number is more than 2k (if trades number is less than 2k -> nextPage: false):

{
  "jsonrpc": "2.0",
  "result": {
    "nextPage": 1542191602000, // trades number is less than 2k -> `nextPage: false`
    "res": [
      {
        "symbol": "tUSTUSD",
        "buyingWeightedPrice": 1.12345,
        "buyingAmount": 12345,
        "sellingWeightedPrice": 1.12345,
        "sellingAmount": -12345,
        "cumulativeWeightedPrice": 1.12345,
        "cumulativeAmount": 12345
      }
    ]
  },
  "id": null
}

---

Basic changes:

• Adds getWeightedAveragesReport method to the main service
• Adds getWeightedAveragesReportCsv method to the main service
• Adds corresponding test coverage

[ezewer]

Looks good to me!