Bug 30205: Add subtitle to subscription detail page

Bug 30205: Add subtitle to subscription detail page

To test:
1. Make some new serial subscriptions and connect them to biblios that contain subtitles.
2. Do a serial search that will return your newly created subscription.
3. Click on one of those subscription to be taken to the subscription detail page.
4. Look at the main heading (h1) and the "Biblio:" line. Notice there is no subtitle in either place
5. Apply patch, restart_all, and reload the subscription detail page.
6. You should now see the subtitle on both the main heading (h1) and on the 'Biblio:' line.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>