REST API v2

- add JWT authentication
- admins are now stored inside the data provider
- admin access can be restricted based on the source IP: both proxy
  header and connection IP are checked
- deprecate REST API CLI: it is not relevant anymore

Some other changes to the REST API can still happen before releasing
SFTPGo 2.0.0

Fixes #197

sftpgo.json

@@ -121,7 +121,6 @@
     "sslmode": 0,
     "connection_string": "",
     "sql_tables_prefix": "",
-    "manage_users": 1,
     "track_quota": 2,
     "pool_size": 0,
     "users_base_dir": "",
@@ -153,7 +152,6 @@
     "templates_path": "templates",
     "static_files_path": "static",
     "backups_path": "backups",
-    "auth_user_file": "",
     "certificate_file": "",
     "certificate_key_file": ""
   },