[Backport 3.2.x][Fixes #7355] Introduce the adoption of groups to define users permissions, and define add_resource permission for users

[marthamareal]

• Creates a new user group contributors with permission base_addresourcebase.
• Updates existing users to belong to a new contributors group.
• Includes user's group perms to get_user_perms function

Checklist

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[lgtm-com]

This pull request introduces 1 alert when merging 7f76821 into 6495e9f - view on LGTM.com

new alerts:

• 1 for Unused import

[afabiani]

@marthamareal LGTM complains about a new unused import introduced with this PR, please, take a look. Thanks.

[giohappy]

@marthamareal let's use lowercase for GeoNode permission names, so please change ADD_RESOURCE to add_resource.