html-css-review: Switch from `em`/`strong` elements to classes

LMS base styles can't be relied upon to provide the generally related styles, bolded and italicized text respectively, for these elements

static/css/mentoring.css

@@ -12,3 +12,10 @@
     color: #9F6000;
     background-color: #FEEFB3;
 }
+
+.mentoring .bold {
+    font-weight: bold;
+}
+.mentoring .italic {
+    font-style: italic;
+}

templates/html/tip_group.html

 <div class="quizz-tip">
   <strong>
-    To the question <em>"{{ self.question }}"</em>,
+    To the question <span class="italic">"{{ self.question }}"</span>,
     {% if submission %}
-      you answered <em>"{{ submission_display }}"</em>.
+      you answered <span class="italic">"{{ submission_display }}"</span>.
     {% else %}
       you have not provided an answer.
     {% endif %}