stop truncating discussion entry notification messages.

fixes CNVS-3949

when sending email notifications re: a new discussion entry,
include the whole entry instead of a truncated, 200
character version.

test plan:
  * create a new discussion post in a class with at least
    two users;
  * set the users' "discussion entry" notification policies
    to ASAP;
  * as user A, post a discussion entry to the previously
    created topic that is longer than 200 characters;
  * as a site admin, navigate to /users/:id/messages, where
    :id is the user ID of user B;
  * verify that user B has received a notification of user
    A's entry and that the notification contains the entire
    body of user A's entry (i.e. it has not been truncated).

Change-Id: I2b524ea5a36cf8e142522d8498e4aba80b8aa554
Reviewed-on: https://gerrit.instructure.com/17789
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Berry <ericb@instructure.com>
Tested-by: Eric Berry <ericb@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>

app/messages/new_discussion_entry.email.erb

@@ -8,7 +8,7 @@
 
 <%= t :body, "%{user} posted a new comment on the thread %{discussion_topic} for %{course}:", :user => asset.user.short_name, :discussion_topic => asset.title, :course => asset.context.name %>
 
-<%= strip_and_truncate(asset.message, :max_length => 200) %> 
+<%= strip_tags(asset.message) %>
 
 <%= t :details, "Join the conversation here:" %>
 <%= content :link %>