Add foreign key on event

#4 (closed) was closed, but some fields still need foreign key on event:

type Transfer @entity {
  blockNumber: Int! @index
→ event (not sure if needed since we already copied the timestamp)

type ChangeOwnerKey @entity {
  blockNumber: Int!
→ event

type Cert @entity {
  createdOn: Int!
→ event (createdIn)
  expireOn: Int!
→ optional event (expireIn)

type SmithCert @entity {
  createdOn: Int!
→ event (createdIn)

Checklist

• transfer
• change owner key
• cert creation, update
• cert expire
• smith cert creation