Add support for client-specific API keys

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

api/v1/require_post_method.php

@@ -5,4 +5,11 @@ if($_SERVER['REQUEST_METHOD'] !== "POST"){
   $return_arr['message'] = "Can only send POST requests to this endpoint.";
   echo json_encode($return_arr);
   exit();
+}
+
+// Client ID must be specific for INSERT/UPDATE/DELETE queries
+// If this API key allows any client, set $client_id to the one specified, else leave it
+if($client_id == 0){
+  //
+  $client_id = intval($_POST['client_id']);
 }