Make Grafana easier to reverse-proxy

**What would you like to be added**: Relative URLs for all redirects, resources etc. **Why is this needed**: Easier reverse proxying. Currently reverse-proxying Grafana is painful, compared to other products. One apparently has to set [domain, path](https://grafana.com/tutorials/run-grafana-behind-a-proxy) - and this has to be synced between Grafana configuration and reverse proxy configuration. The first hurdle is logging in - 302 redirect goes to `/login`. Preferably, it would go to `login` instead. There might be other resources and redirects that need fixing, too. As an example of a product that was fixed in this regard and now is very easy to reverse-proxy, see https://support.zabbix.com/browse/ZBX-4986 . There have been other, possibly similar issues (#10210, #21400).