Add user groups and automatically associate users to groups upon signup

[JustShah]

Warning

Should be applied together with a second PR for the user group assignment via API and a PR in Starter Frontend

User Groups Feature

This pull request introduces a comprehensive User Groups management system in Admin, aimed at enhancing user management and improving the organization of users within the application. The main changes include the ability to enforce default user group assignments upon signup, manage user groups, and improve the admin interface for managing users and their groups.

Included Issues:

#1
#6

Key Features:

1. Enforce User Group Assignment Upon Signup:

   • Updated the UsersController to assign a default user group to new users, based on the store's setting for enforce_group_upon_signup.
   • Added a checkbox to the store settings form to allow administrators to enable or disable this feature.

2. User Groups Management in Spree Admin:

   • Introduced a new UserGroupsController to manage user groups within the admin panel.
   • Added user group management views.
   • Enhanced the user form to allow the assignment of user groups to users.
   • Updated store form to include a selection for the default cart user group.

3. User Group Model and Core Associations:

   • Added Spree::UserGroup model with associations to users and stores to allow flexible user group management.
   • Added necessary database migrations to create the spree_user_groups table and reference user groups in the users and stores tables.

Benefits:

• Improved User Management: This feature provides an efficient way for administrators to organize users into groups, streamlining management and enhancing the customer experience.
• Customizable User Group Assignment: With the option to enforce user group assignment upon signup, store owners can ensure that all new users are categorized appropriately from the start.
• Enhanced Admin Interface: The addition of user groups to the admin menu and interface makes it easier for admins to manage and assign users to groups.
• Flexible and Scalable: The model and association changes allow for further customization, enabling targeted pricing, promotions, or content based on user groups.

Visual Changes:

[fthobe]

@JustShah I removed this as it is not true
Note:
This changes has an dependent PR on front-end S3-Store/s3_starter_frontend#1

[fthobe]

@JustShah Add mechanism to add new users to default group