Add support for client-specific API keys

Refactoring API. Added a contact update endpoint.
Small misc changes.

api/v1/require_get_method.php

@@ -5,4 +5,9 @@ if($_SERVER['REQUEST_METHOD'] !== "GET"){
   $return_arr['message'] = "Can only send GET requests to this endpoint.";
   echo json_encode($return_arr);
   exit();
+}
+
+// Wildcard client ID for most SELECT queries
+if($client_id == 0){
+  $client_id = "%";
 }