[Incidents Page] Implement configuration to show test incidents

[miguelsimoes]

Currently, the incidents page shows all incidents that are registered without the possibility of filtering by whether they are (or not) test incidents.

Would like to request to:

• By default filter out test incidents
• Add option to enabled retrieval of test incidents, like for example:
  rootly: rootly-main: isDefault: true proxyPath: /rootly/api rootly-sandbox: proxyPath: /rootly-sandbox/api showTestIncidents: true
  So that we can retrieve those incidents in an account we want to (but by default hide them from being retrieved)