Migrate the plugin to React

[rozetko]

Migrated from Angular to React:

• Config page to React
• Kentik Datasource
• Call to Action panel

Removed:

• Device List page
• Kentik Home: Device List
• Add Device page
• Device Configuration page
• Root page

Changes:

• change minimal supported Grafana version from 8.0.0 to 8.3.0
• rename Call to Action panel to Description
• Query Editor: swap Site and Device selectors

Other changes:

• move alert_helper to utils/

Notes:

• in order to update left-side menu and Kentik Home dashboard you'll need to:
  • disable the plugin and then enable it again (Configuration -> Plugins -> Kentik Connect Pro -> Disable -> Enable)
  • restart Grafana