add frontend piwik tracking code

[achen2401]

Add PIWIK Tracking

• UserId for PIWIK tracking is retrieved via user JWT IDToken, see screenshot (note the key name), for reference, see here via call to AuthConfig API

• Site ID for PIWIK tracking is retrieved via environment variable set in either appsettings.json (under key client) or env.json. (TBD) The code will look in either one of these regardless. Any one of those variables will work:

  • REACT_APP_MATOMO_SITE_ID
  • MATOMO_SITE_ID
  • REACT_APP_PIWIK_SITE_ID
  • PIWIK_SITE_ID
  • SITE_ID

• The PIWIK tracking code will not run if either userId or siteId` is null/undefined

Note If adding piwik site Id under the key client in appsettings.json changing code is required see here for the new key-value pair to be returned.
Note If using env.json, new script (like this) is required to populate it with environment variables needed by the frontend

[rpkyle]

I'd cast my vote for using env.json, if it helps avoid further edits to Leaf code. I also favor the position of not inserting elements unrelated to Leaf into appsettings.json, if we can avoid it.