Improves conversation view by providing headings

fixes CNVS-16990

Switched b and p tags to h2 and h3 tags as specified by the ticket.

Test plan:
* Log in as any type of user and navigate to the conversations view
* Ensure that you can navigate the new header hierarchy of the messages
  list and the messages detail view using a keyboard and screenreader
* Ensure that when you open a message via the keyboard the message
  subject gets focus

Change-Id: I89e780fdda4308656627c0da58e81b5e3dc21cb3
Reviewed-on: https://gerrit.instructure.com/45169
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Reviewed-by: Steven Shepherd <sshepherd@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>