Use server-host / DRONE_HOST as source for the public domain

[anbraten]

Currently the request address being used to access the webinterface is for example used as base for webhooks and some links.

This is done by using httputil.GetURL(c.Request). Maybe it would be nice to use the DRONE_HOST string for these places. Currently DRONE_HOST is only used for the automatic letsencrypt setup.

Using the domain a user requested the webinterface with brings some problems:

• if you use multiple domains to access the webinterface the current domain would be used instead of a specific one
• you can not develop with localhost => would for example register webhooks to github, ... with localhost which is denied