Replace .yml value from DMPOnline to DMP Assistant

The corresponding value for `devise.mailer.confirmation_instructions.subject` in `config/locales/translation.fr-CA.yml` specifies "Assistant PGD", but maybe it should be verified as well.

Note: I don't think this is the proper way to update the locales files. Rather, I believe only `config/locales/en.yml` should be directly modified. However, #920 is currently preventing that.

config/locales/translation.en-CA.yml

@@ -232,7 +232,7 @@ en-CA:
       invited: You have a pending invitation, accept it to finish creating your account.
     mailer:
       confirmation_instructions:
-        subject: Confirm your DMPonline account
+        subject: Confirm your DMP Assistant account
       reset_password_instructions:
         subject: Reset password instructions
       unlock_instructions: