Tailor signature certificate email copy for approvals

Store the per-signature email subject as text on the electronic_signatures
row at creation time, mirroring the consent_text pattern. The document
approval service sets "Your approved <Title> - Certificate of Completion";
other callers default to "Your signed <Name> - Certificate of Completion".
The certificate worker uses signature.email_subject as the email subject,
falling back to the default format when the column is empty.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

packages/emails/src/ElectronicSignatureCertificate.tsx

@@ -21,13 +21,11 @@ import EmailLayout, {
 
 export const ElectronicSignatureCertificate = () => {
   return (
-    <EmailLayout
-      subject={`Your signed ${"{{.DocumentName}}"} — Certificate of Completion`}
-    >
+    <EmailLayout subject={'{{.Subject}}'}>
       <Text style={bodyText}>
-        Your <strong>{"{{.DocumentName}}"}</strong> has been signed
-        electronically. A Certificate of Completion is attached to this email
-        as a PDF document.
+        A Certificate of Completion for{" "}
+        <strong>{"{{.DocumentName}}"}</strong> is attached to this email as a
+        PDF document.
       </Text>
 
       <Text style={bodyText}>