Missing Backup Records Change

Changed the way the missing backup records function works, removed all
html etc from the class and moved it to the page template. Also made the
 dialog to show missing records visible on the backups landing page as
 it was hidden away beforehand.
Session details update

Allows user to update some missing information from the add new backup
page for ease of use. WIP
Merge branch 'master' into Backup-Layout
Finish Ajax session update via backup record.

Allows user to update missing session details from the backup page and
updates the list of backup drives that can be selected if a composer is
added.

Also a tidy up of the layout within a the backup php pages. I was
a lot messier when I wrote this originally!