Multiple contacts connected to a single user

Problem/Motivation

When "Connect users to RedHen contacts" is checked, it's possible to accidentally connect multiple RedHen Contacts to a single Drupal User.

Steps to reproduce

1. Create a contact connected to a user with the email test@example.com
2. Then run the following code:

  $contact = \Drupal\redhen_contact\Entity\Contact::create([
    'type' => 'general',
    'email' => 'test@example.com',
  ]);
  $contact->save();

3. The duplicate contact is created and attached to the user account test@example.com, resulting in two contacts connected to the same user.

Proposed resolution

Patch below.

Remaining tasks

User interface changes

API changes

Data model changes