Alertmanager URI might contain basic auth username & password, we should replace password with 'xxx' in logs and and error messages.
Go will still print it in HTTP request errors, but that will be fixed in the next Go release - https://go-review.googlesource.com/c/go/+/102855Fixes#259
Fixes#190.
With this feature unsee can be configured to proxy requests to selected Alertmanager instances, if it's enabled unsee silence form will send a request via unsee rather than directly. This allows users to manage silences in environments where they have access to unsee but not to Alertmanager. Only silences endpoints on Alertmanager API are proxied.
