Fix container height in message detail screen

// FREEBIE

stylesheets/_conversation.scss

@@ -45,6 +45,9 @@
 }
 
 .message-detail {
+  .container {
+    height: calc(100% - (#{$header-height} + 4px));
+  }
 
   .message-container {
     background: white;

stylesheets/manifest.css

@@ -571,6 +571,8 @@ input.search {
     font-family: monospace;
     padding: 0 1em; }
 
+.message-detail .container {
+  height: calc(100% - (36px + 4px)); }
 .message-detail .message-container {
   background: white;
   padding: 1em 0; }