Validate group data on user save API call

[snipe]

We've been seeing some funky errors in Rollbar from folks who are passing weird (invalid) data to the user's create endpoint. This should validate that the group exists before trying to save it, which should cut them off via validator if the group data is funky.

This would fix the PDOException: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'accessories.checkin' in 'field list' in /snipe-it/vendor/laravel/framework/src/Illuminate/Database/MySqlConnection.php:39 errors in RB.

[what-the-diff]

PR Summary

• Enhanced the User Authorization for Creating Users
  The authorize method in SaveUserRequest.php was modified to include an additional check. This check is to ensure that the user attempting to create a new user actually has the permission to do so. This makes the operation of user creation more secure.

• Inclusion of Group Validation Rule
  In the SaveUserRequest.php file, an additional rule for groups was incorporated within the rules method. This means that whenever a new user is being created, an extra validation process will make sure the 'groups' data provided follows the correct standard.

• Feature Tests for User Creation added
  A new file, StoreUserTest.php, has been created which contains comprehensive tests for the user creating API. This involves checking if the correct permissions exist for creating a user, verification of successful user creation when valid data is provided, and an investigation into the rejection of invalid group data. This ensures a smoother and more reliable overall user creation process.

[snipe]

Looks like that tighter gate broke some tests - will fix when I get back from lunch.