feat(sqllab): extra logging when chart is downloaded

[zephyring]

SUMMARY

1. add logging for when chart is downloaded as image, json, csv, xls
2. it will call the superset/log endpoint with form_data payload like

[{
  "source":"explore", 
  "source_id":80,
  "event_name":"chart_download_as_json",
  "chartId":80,
  "chartName":"a chart name",
  "event_type":"user"
  ...
}]

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API