Restrict adding users to members list if already subscribed to another plan

[reyraa]

Description

With the current block code, users can add any other users to the members list of their subscription. This might be problematic for users who already have a more desirable subscription.
To prevent inconvenience, we should only allow users to add other users to the members list of their subscriptions if they are not already a member of another subscription.

Acceptance Criteria

• Users can only be a member of one subscription at a time.
• Users should be able to update the members list of their subscription only if all new users are not members of another subscription.
• The transaction should fail if at least one of the new members are already a member of another subscription.
• Sufficient unit test is required.