Substitute visible labels for placeholders in post submission form. Fix alignment.

TNL-5166

lms/static/sass/discussion/_discussion.scss

@@ -8,6 +8,7 @@ body.discussion {
     @include clearfix();
     box-sizing: border-box;
     width: 100%;
+    padding-top: 0;
 
     h1 {
       font-size: $forum-x-large-font-size;
@@ -19,14 +20,12 @@ body.discussion {
 
     .post-cancel {
       @include float(left);
-      margin: ($baseline/2) 0 0 ($baseline*0.75);
+      @include margin($baseline/2, 0, 0, $baseline*0.75);
     }
 
     .post-update {
       @include float(left);
       margin-top: ($baseline/2);
-      padding-bottom: ($baseline/10);
-      height: 37px;
 
       &:hover, &:focus {
         border-color: #222;
@@ -350,7 +349,6 @@ section.discussion {
 
 .new-post-article {
   display: none;
-  margin-top: $baseline;
 
   .inner-wrapper {
     max-width: 1180px;

lms/static/sass/discussion/_layouts.scss

@@ -31,4 +31,8 @@
 
 .discussion-column {
     min-height: 500px;
+
+    .new-post-article {
+        margin-top: -$baseline;
+    }
 }

lms/static/sass/discussion/views/_create-edit-post.scss

@@ -25,7 +25,9 @@
         width: 100%;
         vertical-align: top;
         border-width: 0;
-        padding: 0 ($baseline/2);
+        &:not([type="text"]) {
+          padding: 0;
+        }
       }
 
       .field-label-text {
@@ -49,6 +51,9 @@
       width: 50%;
       font-size: $forum-small-font-size;
 
+      &#new-post-editor-description {
+          @include padding-left(0);
+      }
     }
   }
 
@@ -100,6 +105,11 @@
     color: $gray-d3;
     font-weight: 600;
     line-height: 36px;
+    @include float(left);
+
+    &:last-of-type {
+      @include float(right);
+    }
 
     .icon {
       @include margin-right($baseline/4);