add reply messaging to email notifications

fixes CNVS-5793

test plan:
  * set a user's notification settings to ASAP;
  * as another user, do the following and verify that
    the email received contains messaging informing the
    user that they can reply directly to the email:
  * send a conversation message;
  * comment on a discussion topic that the first user is
    subscribed to;
  * comment on one of the user's assignment submissions.

Change-Id: Iffe0763aad26579e5b5955f4f31e5e5242c14355
Reviewed-on: https://gerrit.instructure.com/29504
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>

app/messages/conversation_message.email.erb

@@ -18,7 +18,7 @@ Subject: <%= asset.conversation.subject %>
 <% if asset.has_media_objects? %>
 <%= t :audio_comment, "This message includes media comments. To listen or reply, click the link below:" %>
 <% else %>
-<%= t :reply_message, "To reply to this message, click the link below:" %>
+<%= t :reply_to_conversation_message, "You can reply to this message in Canvas by replying directly to this email." %>
 <% end %>
 
 <%= content :link %>

app/messages/conversation_message.email.html.erb

@@ -34,7 +34,7 @@
 <% if asset.has_media_objects? %>
 <p><%= t :audio_comment, "This message includes media comments. To listen or reply, click the link below:" %></p>
 <% else %>
-<p><%= t :reply_message, "To reply to this message, click the link below:" %></p>
+<p><%= t :reply_to_conversation_message, "You can reply to this message in Canvas by replying directly to this email." %></p>
 <% end %>
 
 <p><a href="<%= content :link %>"><%= content :link %></a></p>

app/messages/new_discussion_entry.email.erb

@@ -10,5 +10,5 @@
 
 <%= html_to_simple_text(asset.message, :base_url => "#{HostUrl.protocol}://#{HostUrl.context_host(asset.context)}") %>
 
-<%= t :details, "Join the conversation here:" %>
+<%= t :discussion_entry_reply_message, "Join the conversation using the link below, or comment by replying to this message." %>
 <%= content :link %>

app/messages/new_discussion_entry.email.html.erb

@@ -25,5 +25,6 @@
     background-image:    -moz-linear-gradient(left, #ddd, #aaa, #eee);
     background-image:     -ms-linear-gradient(left, #ddd, #aaa, #eee);
     background-image:      -o-linear-gradient(left, #ddd, #aaa, #eee);" />
-
-<p><a href="<%= content :link %>"><%= t :details, "Click here to join the conversation" %></a></p>
+<p>
+<%= mt(:discussion_entry_link_message, "[Click here to join the conversation](%{url}), or comment by replying to this message.", url: content(:link)) %>
+</p>

app/messages/submission_comment.email.html.erb

@@ -32,7 +32,7 @@
 <% if asset.media_comment? %>
 <p><%= t "#messages.conversation_message.email.audio_comment", "This message includes media comments. To listen or reply, click the link below:" %></p>
 <% else %>
-<p><%= t :link_message, "You can review the submission details here:" %></p>
+<p><%= t :submission_comment_link_message, "You can review the submission details using the link below, or can reply to this comment by responding to this message." %></p>
 <% end %>
 
 <p><a href="<%= content :link %>"><%= content :link %></a></p>

app/messages/submission_comment_for_teacher.email.erb

@@ -13,7 +13,7 @@
 <% if asset.media_comment? %>
 <%= t "#messages.conversation_message.email.audio_comment", "This message includes media comments. To listen or reply, click the link below:" %>
 <% else %>
-<%= t :link_message, "You can review the submission details here:" %>
+<%= t :submission_comment_link_message, "You can review the submission details using the link below, or can reply to this comment by responding to this message." %>
 <% end %>
 
 <%= content :link %>