Increase color contrast for forum features

The icons and text for pinning and flagging ("Report Misuse") had
contrast ratios that did not meet the minimum WCAG requirement of 4.5.
This fixes FOR-200.

lms/static/sass/_discussion.scss

@@ -2619,7 +2619,7 @@ body.discussion {
   width: 10px;
   height: 14px;
   padding-right: 3px;
-  color: #aeaeae;
+  color: #666;
 }
 
 .pinned .icon {
@@ -2633,14 +2633,14 @@ body.discussion {
 }
 
 .pinned span {
-  color: #B82066;
+  color: $pink;
   font-style: italic;
   //cursor change is here since pins are read-only for inline discussions.
   cursor: default;
 }
 
 .notpinned span {
-  color: #888;
+  color: #666;
   font-style: italic;
 }
 
@@ -2665,7 +2665,7 @@ display:none;
     
 .notflagged .icon {
   display: block;
-  color: #aeaeae;
+  color: #666;
   float: left;
   margin: 3px;
   width: 10px;
@@ -2676,7 +2676,6 @@ display:none;
 .flagged .icon
 {
   display: block;
-  color: #aeaeae;
   float: left;
   margin: 3px;
   width: 10px;
@@ -2686,11 +2685,11 @@ display:none;
 }
 
 .flagged span {
-  color: #B82066;
+  color: $pink;
   font-style: italic;
 }
 
 .notflagged span {
-  color: #888;
+  color: #666;
   font-style: italic;
 }