Implement mixpanel for `crm/contacts`

[freaz]

Description

It is a bit controversial, because Mixpanel isn't actually CRM and doesn't allow creating new contacts (users). You as developer need to send unique id of the user, and if it doesn't exists it will be created.

Also Mixpanel doesn't does not expose users filtering in public API.

It share provider from https://github.com/superfaceai/station/pull/158

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.