conversations: handle shift-click to select message ranges

fixes CNVS-10322

test plan:
  * open conversations
  * select a conversation
  * shift-click another conversation
  * verify that the two conversations and all between them
      are now selected
  * verify that there isn't a flash of text selection

Change-Id: Ia1a7fbf154dec68b343243eddf0b1ccf93adc490
Reviewed-on: https://gerrit.instructure.com/28719
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>