collapse multiple hats to a single signer -- if the same email address is signing in multiple roles, give them the same esnum

[jobchong]

From @mengwong on September 28, 2015 13:51

right now we send two emails and they are confusing. people sign the first one and gmail collapses the second so they don't see it.

Copied from original issue: legalese-io/legalese-google-app#9