reset conversation context on dialog close

test plan:
* create two courses, e.g. "Course A" and "Course B"
* send a message to all the members of a subset of "Course A"
 e.g. "Course A: Teachers"
* without refreshing the page, open the dialog to send another
message, this time to all the members of a subset of "Course B"
* should accurately show "Course B: Teachers", instead of
 the first course

closes #CNVS-14969

Change-Id: I2af373cdd4292c93aba944936573778679290d0b
Reviewed-on: https://gerrit.instructure.com/42163
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>