add vertical scrollbar to scheduler appointments.
add vertical scrollbar to scheduler appointments.

fixes CNVS-5037

when viewing a scheduler appointment detail popup as a
student, some content would not be visible if the
appointment had a description and > 5 students. now the
content inside of the popup scrolls vertically.

test plan:
  * as a teacher, create an appointment group with the
    'allow students to see who has signed up for time
    slots' option selected and a description > 5 lines;
  * as five different students in the course, sign up for
    the appointment group;
  * as one of those students student, click the appointment
    group in the scheduler week view to open the details
    popup;
  * verify that the details popup content is scrollable and
    is all visible.

Change-Id: I32d7584b11e76f76fb7ed5e8345fee9121e675ad
Reviewed-on: https://gerrit.instructure.com/19275
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jon Willesen <eb5ce5e6a76c243326fecfa94534101600f87bd9@instructure.com>
QA-Review: Clare Hetherington <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Product-Review: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>

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