Store private ticket fields (ex: the originators email address) as their
SHA1 hash so that malefactors cannot read them.  Add the new "concealed"
table to the repository database and store mappings from SHA1 hashes back
to email addresses in that table.  Ticket [a24ec6005f].  Note: run
"rebuild" on repositories after updating to this version of fossil in order
to create the "concealed" table.  Need to add the ability to manage the
concealed table from the web interface and the ability to sync concealed
content between trusted repositories.