Update "comment" notification email to add unsubscribe link

We modified the link that previously redirected us to the "My content"
page to redirect us to the new page for managing subscriptions.

We also adapted the existing generic text by adding a description of
the related notification.

app/mailers/mailer.rb

@@ -9,6 +9,7 @@ class Mailer < ApplicationMailer
     @comment = comment
     @commentable = comment.commentable
     @email_to = @commentable.author.email
+    manage_subscriptions_token(@commentable.author)
 
     with_user(@commentable.author) do
       subject = t("mailers.comment.subject", commentable: t("activerecord.models.#{@commentable.class.name.underscore}", count: 1).downcase)
@@ -150,4 +151,9 @@ class Mailer < ApplicationMailer
         mail.perform_deliveries = false
       end
     end
+
+    def manage_subscriptions_token(user)
+      user.add_subscriptions_token
+      @token = user.subscriptions_token
+    end
 end