add User Display Names

[josh-chamberlain]

Context

We'd like to display annotations and submissions leaderboard-style, without displaying anyone's email address. We should give people the ability to add a display name.

Requirements

• Create this property at the user level.
• Description should be something like "Required to be featured on leaderboards. Should not use vulgar or hateful language."
• Allow letters, numbers, -, _ (no spaces or other characters).
• Must be between 3–30 characters
• Must not be taken by another user (case insensitive for this check)
• Allow it to be added via the API upon creation or after the fact, or updated by the user any time.
• Give helpful errors if the requirements are not met.
• Consider storing past display names for each user, and limiting changes to once every 7 days.
• Use the user's numeric ID as a default display name, if none is added

Followup

• add Display Name to User Profile pdap.io#374