Add a link on the login pages to switch between admin and web client login

The links are hidden if only the web admin or only thw web client is
enabled and can also be controlled using the "hide_login_url" setting

Fixes #485

sftpgo.json

@@ -191,7 +191,8 @@
         "enable_https": false,
         "client_auth_type": 0,
         "tls_cipher_suites": [],
-        "proxy_allowed": []
+        "proxy_allowed": [],
+        "hide_login_url": 0
       }
     ],
     "templates_path": "templates",