accessibility for new conversation dialogs

fixes CNVS-14515

test plan
- as a teacher, visit /conversations
- make sure your inbox is set to display 'All Courses'
- using screenreader, compose a new message
- verify that no course is selected
- verify that the 'To' autocomplete fields are disabled
- select a course
- verify that the 'To' autocomplete fields are no longer
  disabled and are available to the screenreader
- celebrate! :tada:
- go back to your inbox
- filter messages by one course
- compose a new message
- verify that the course you filtered for is selected
- verify that the 'To' autocomplete fields are not disabled
- reply to an existing message, verify that the 'To' field is accessible

- as an admin, visit /conversations
- verify that 'To' is *not* disabled for screenreaders

Change-Id: I2fddc876c881dda87ddc06e8e39a31b37bedf1d9
Reviewed-on: https://gerrit.instructure.com/40781
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>