[feat] Add header authentication support

In some cases there are needs to authenticate user not in Taskcafe itself.
For this reason option server.remote_user_header was added.

```toml
[server]
remote_user_header = "X-Remote-User"
```

With turned on Taskcafe listens X-Remote-User http header and skip
password checking. But still check user existence and activity flag.

conf/taskcafe.example.toml

@@ -1,5 +1,6 @@
 [server]
 hostname = '0.0.0.0:3333'
+remote_user_header = ""
 
 [email_notifications]
 enabled = true