Change endorsement UI for forum question threads

Responses to question threads can be "marked as answer" instead of
"endorsed".

lms/static/sass/discussion/_discussion.scss

@@ -656,6 +656,10 @@ body.discussion {
       background: url(../images/endorse-icon.png) no-repeat;
     }
 
+    &.mark-answer .check-icon {
+      background: url(../images/answer-icon.png) no-repeat;
+    }
+
     &.is-endorsed {
       border: 1px solid #4697c1;
       @include linear-gradient(top, #6dccf1, #38a8e5);
@@ -664,6 +668,11 @@ body.discussion {
       .check-icon {
         background-position: 0 -12px;
       }
+
+      &.mark-answer {
+        @include linear-gradient(top, tint(#1d9348, 60%), tint(#1d9348, 20%));
+        border: 1px solid #1d9348;
+      }
     }
   }
 

lms/templates/discussion/_underscore_templates.html

@@ -153,7 +153,20 @@
 <script aria-hidden="true" type="text/template" id="thread-response-show-template">
     <header>
         <a href="#" class="vote-btn" role="button" aria-pressed="false"><!-- DiscussionContentView.renderVote() populates this --></a>
-        <a href="javascript:void(0)" class="endorse-btn action-endorse" style="cursor: default; display: none;" data-tooltip="${_('endorse') | h}"><span class="check-icon" style="pointer-events: none; "></span></a>
+        <%
+            tooltip_expr = u"<%- thread.get('thread_type') == 'question' ? '{mark_answer}' : '{endorse}' %>".format(
+                endorse=_("endorse"),
+                mark_answer=_("mark as answer")
+            )
+        %>
+        <a
+            href="javascript:void(0)"
+            class="endorse-btn action-endorse ${"<%= thread.get('thread_type') == 'question' ? 'mark-answer' : '' %>"}"
+            style="cursor: default; display: none;"
+            data-tooltip="${tooltip_expr}"
+        >
+            <span class="check-icon" style="pointer-events: none; "></span>
+        </a>
         ${"<% if (obj.username) { %>"}
         <a href="${'<%- user_url %>'}" class="posted-by">${'<%- username %>'}</a>
         ${"<% } else { %>"}