Implement managed users feature

A managed user is authenticated externally to Sentry, for example
via basic auth by the web server. Such users should not be allowed
to change their username or password from within Sentry.

A new configuration setting, SENTRY_MANAGED_USER_FIELDS, can be
used to further prevent a managed user from changing their name
or email address.
Merge pull request #1255 from jaysoffian/managed_user

Implement managed users feature