Bug 19981: Switch single-column templates to Bootstrap grid: Course reserves

Bug 19981: Switch single-column templates to Bootstrap grid: Course reserves

This patch updates three single-column course reserves templates to use
the Bootstrap grid:

- course-reserves.tt - The main Course Reserves page
- course-details.tt - The detail view of an individual course
- course.tt - Editing a course
- add_items-step1.tt - Adding items to a course, step 1 (scan barcode)
- add_items-step2.tt - Adding items to a course, step 2
- invalid-course.tt - The error page shown if you try to view
  course_reserves/course-details.pl directly without passing a course id
  in the URL
  - Also changed in this template: Error message has been wrapped in the
    standard "dialog alert" <div>.

Each of these pages should look correct, with a single centered column
with wide margins on either side. At lower browser widths the margins
should disappear.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>