feat: bidirectional link user-accounts <-> CRM-contacts

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Proposal

• Auto-add a CRM-contact to an issue when the user_email (or public_email) of the user-account who created the issue, ie. the author, matches some contact_email(s) in the relevant group;
• When the author is the gitlab-support-bit (e.g. it's a service desk email) and the external_email matches a user_email/public_email of some user-account, set this user as issue's author.

A more ambitious goal would be to bidirectional link user-accounts <-> CRM-contacts, if their email addresses match, ie:

• display matching CRM-contact(s) on User-account pages & vice versa,
• annotate avatars with a decoration to denote there is a linked contact for it,
• GraphQL API/DB table changes,
• etc.

Configuration

A boolean project "master" switch would enable/disable this automation, potentially overridden per user-account and/or contact. Another potential config may regard which user email to consider in the matching(main, public, all).

Use-cases

1. We usually know our customer contact-infos from side channels, but we cannot reflect them on the 3rdp users interacting daily with gitlab.

2. With Service Desk we need to see which (if any) user-account is involved when we receive an email.

History

This is part of a more general goal, to automatically link user-accounts <-> CRM contacts (aka "clients"?), as @leetickett's described it in x2 comments [of #2256] and gave me the ok to open this issue.