bootstrap improvements, fixes #1392

- remove unused styles (no IDs called preview or comments)
- replace margins & paddings with bootstrap utility classes
- apply min body height to placeholder, plaintext & prettymessage, to move footer to end of viewport

css/bootstrap/privatebin.css

@@ -87,8 +87,8 @@ body.loading {
 	word-wrap: break-word;
 }
 
-#preview {
-	margin-bottom: 10px;
+#message, #placeholder, #plaintext, #prettymessage {
+	height: 70vh;
 }
 
 #message, .replymessage {
@@ -100,10 +100,6 @@ body.loading {
 	margin: 5px 0;
 }
 
-#comments, #comments button {
-	margin-bottom: 10px;
-}
-
 .comment {
 	border-left: 1px solid #ccc;
 	padding: 5px 0 5px 10px;