conversations: add batch "mark as read"

fixes CNVS-10728

test plan:
  * open new conversations
  * select multiple unread conversations with shift-click
    (or select multiple and mark as unread)
  * in the header's "More options" menu, click "mark as read"
  * verify that the messages are marked as read
  * reload the page
  * verify that the change persists

Change-Id: I21cad13d1334f15708fbf0a7a4fdbba51304e487
Reviewed-on: https://gerrit.instructure.com/31700
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>