Clean up font weights

lms/static/sass/discussion/elements/_labels.scss

@@ -4,8 +4,8 @@
 .discussion {
   .post-label {
     @include margin($baseline/4, $baseline/2, 0, 0);
+    @extend %t-light;
     font-size: $forum-small-font-size;
-    font-weight: 100;
     display: inline;
     white-space: nowrap;
 

lms/static/sass/discussion/views/_thread.scss

@@ -22,6 +22,7 @@
 .discussion-article {
     .posted-details {
         @extend %t-copy-sub2;
+        @extend %t-light;
         margin: ($baseline/5) 0;
         color: $forum-color-copy-light;
 
@@ -160,8 +161,8 @@
     // post context
     .post-context {
         @extend %t-copy-sub2;
+        @extend %t-light;
         color: $forum-color-copy-light;
-        font-weight: 100;
 
         // CASE: no courseware context or cohort visibility rules
         &:empty {