Add Tailscale support

[andig]

This PR provides an integrated Tailscale client:

tailscale:
  host: evcc # or empty
  authkey: <tailscale key>

once running, evcc will be available via https from any Tailscale-enabled device (e.g. Mobile phone with Tailscale client installed) via its Tailscale hostname: https://evcc.tsnet4711.tailscale.com

TODO

• [ ] add authentication to ui
• [ ] add build switch