Add UserHandler with promote/demote capabilities.

Fix bug in ConfigLoad preventing salt and hash fields from being populated when loading users.
Convert User to a class.
Fix UserService.getUser bug where it would compare the input user string instead of the resulting user against null.

README.md

@@ -20,7 +20,9 @@ The server includes a (working but very WIP) HTTP API for third-party clients an
 - [x] Search
 - [x] Downloads
   - [ ] Resumable downloads
-- [ ] User Management
+- [x] User Management
+  - [x] Promote/demote users
+  - [ ] Create/delete users
 
 ### Web interface
 - [x] Authentication