Don't explicitly set ForeignKey type

SQLAlchemy will pick it up for us itself, and will deal with making sure
it is the right type.
When creating new revisions/entry lookup userid

We can just set the author field to the user found, and magic, things
get handled for us.
Move from Integer primary keys to UUID
Treat UUID fields from PgSQL as UUID in Py
Stringify before JSON'ification
Check if site ID is a valid UUID
Remove RevisionRendered

This is not necessary.
Create partial unique index on default/site_id

This way the user can only allow a single domain to be set as the
default domain.
Site should return information about site

Not the entries associated with the site.
Change some indexes around ...
If we don't have a rendered version, return raw
Use the post.entry property
Return the first found entry for the site
Add new entries traversal
Site traverses to entries
Import Entries into traversal.v1 space