let the subject of the email be modifiable via the notification template

currently, notification email subjects are not customisable. Since the change to use freemarker templates with notification email, there exists a way to allow email subject customisation in the email template.

The suggested model is to allow the subject be specified on the first line of the email template, after the ftl tag, beginning with the string "Subject: " , e.g.,

[#ftl]
Subject: ${comment.user.displayName} commented in ${review.permaId}       [#-- this is the subject line --]

[#if comment?exists]
    [@compress single_line=true]
    ${review.permaId}
    [#if comment.defectRaised]
        ${comment.user.displayName} raised a defect
    [#else]
        commented
    [/#if]

If the subject line doesn't exist, the default subject is used.