"Permission denied" error in the Kentik dashboards

[corpglory-dev]

Currently only Admins can see the Kentik dashboards. Viewers and Editors get "Permission denied" error when trying to open a Kentik dashboard.

The error happens because the dashboards are trying to read the Kentik datasource settings from Grafana but it can be done only by Admins.

This PR makes it able to open the Kentik dashboards by Viewers and Editors, not only by Admins.

How to make the plugin work after the update

• restart Grafana to update the plugin routes
• go to the plugin config (e.g. http://localhost:3000/plugins/kentik-app/) and click "Update"
• now any type of user should be able to open the Kentik dashboards

Changes

• Plugin Config page: new "invisible" url field which can be used by plugin.json. It's set according to the "Region" type when the "Update" button is clicked
• plugin.json: replace region-specific proxy routes (default/api/v5, eu/api/v5, custom/api/v5) with a generic api/v5/* route. The new route uses the url field from the plugin config
• datasource/kentik_api.ts: use the new route instead of region-specific ones
• remove "Region" usage from all Kentik panels and components
• remove datasource/region_helper.ts as it's not used anymore
• fix tests