Idea: Set the Grafana link default via environment variable

[junderw]

Related: #1893

Use case: When creating a one-click deploy solution for teslamate, one of the biggest hurdles is explaining the separation of teslamate and grafana. It would be better, since I will know the relative port and/or path to grafana, if I could set the grafana link and teslamate link (in the settings) default value via the environment variables.

Solution:

TESLAMATE_URLS_WEBAPP=https://mydomain.com:4000 and TESLAMATE_URLS_DASHBOARDS=https://mydomain.com:3000 will make it so that if there is no setting stored in the config, the default value is set to these environment variables.