Adding group attribute (accepts a group ID) to pmpro_member shortcode

[kimcoleman]

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Changes proposed in this Pull Request:

Added the attribute "group" which accepts a single group ID value so that member info can be shown in the context of a single level group.

Note we still need to show group ID somewhere on the Settings > Levels admin list so that people know what group ID to use.

The logic in this now says:

• If the shortcode doesn't specify level IDs or a group, grab the first level we find for the user.
• If the shortcode has a levels attribute, grab the first level the user has in the that array otherwise return nothing.
• If the shortcode has a group attribute, grab the first level the user has in the that group otherwise return nothing.

How to test the changes in this Pull Request:

1. Give yourself a level in a group with ID 1
2. Add a shortcode like [pmpro_member field="membership_name" group="1"] to a post or page
3. Check the frontend and see your level name.
4. Now, change the shortcode's group ID attribute to 2 or any other group you don't have a level.
5. The frontend should show nothing for your membership.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

• ENHANCEMENT: Added attribute group to the pmpro_member shortcode to show specific member info in the context of a level group.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.