Entity Registration 2.x

- Registration has been reworked to allow configuration for any type of entity to register - not
just users.
- Registration "user" has been renamed "registrant" across the module.
- Registration is now done primarily by email address, for both users and anonymous registrants.

Backwards-incompatible/breaking changes:

- Registration by user id and explicit "anonymous registration" have both been removed/changed. In
2.x, registrant entities are always registered by email address. An lookup is performed for an
entity matching the email address, and if not found, the registration is automatically considered
"anonymous".

- Registration permissions have been simplified. Permissions "Register other accounts" and
"Register other people" have been removed.

- Registration objects no longer have a "user_uid" field. This field has been renamed
"registrant_id" and references an entity with a type configured by the registration type (default
is user). Any implementations of registration hooks or other code that uses registration objects
should be updated to use the new property.

- Registration token named 'user' has been changed to 'registrant' to support registration for
arbitrary registrant entity types. For existing registration types with user registration (default
for all registration types created in 1.x), simply replace all occurrences of registration token
'user' with 'registrant'.
