Fix multiple spaces not beeing displayed in message bubbles

Multiple spaces ("     ") are displayed by browsers as a single
whitespace character if 'white-space: pre-wrap;' is not set.

stylesheets/_conversation.scss

@@ -179,6 +179,8 @@
 
     .content {
       -webkit-user-select: text;
+      white-space: pre-wrap;
+
       a {
         word-break: break-all
       }

stylesheets/manifest.css

@@ -624,7 +624,8 @@ input.search {
       border-bottom: 6px solid transparent; }
     .message-detail .bubble .content,
     .message-list .bubble .content {
-      -webkit-user-select: text; }
+      -webkit-user-select: text;
+      white-space: pre-wrap; }
       .message-detail .bubble .content a,
       .message-list .bubble .content a {
         word-break: break-all; }