[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.

docs/remote-auth.md

@@ -0,0 +1,9 @@
+# Remote authorize
+If you need to authenticate user with some proxy, you should use
+```toml
+[server]
+remote_user_header = "X-Remote-User"
+```
+With this option Taskcafe will take username from
+`X-Remote-User` HTTP header and will not check its password.
+You can use any header you want.