Make users a first-class citizen in Sentry

[dcramer]

We already track users via a plugin. Let's rip the plugin out, and make user tracking first-class.

• Add users_seen (unique users) to Group
• Move the user_emails tag to its own navigation pane (below Similar Events)
• Dedicated page for seeing whats happened historically to a user (vs just visiting the search page)
• The ability to email a specific user, or all users affected by an event.

[jokull]

Would deserve an 🍦

[dcramer]

Merged in a first-pass thats using user_email to track it.

We'll need to make some changes yet:

• track user's by ID (not email), outside of the tag system (mostly because we'd have to special case it otherwise)
• default id from email address if no id present, otherwise ip_address