conversations: compose dialog usability tweaks

fixes CNVS-12625, CNVS-13234

test plan:
  * open conversations

  * open the conversation search user finder
  * select a course
  * verify that a "Back" option is present

  * open the recipient finder and ctrl+click on a result
  * verify that the recipient is added but the finder stays open

  * focus the message body textarea
  * verify that enter and shift+enter both just add a line break

Change-Id: I1d7bdd3c882ead8ccb42ae0bd33d664c08847cec
Reviewed-on: https://gerrit.instructure.com/35264
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>