Set max-width for README, issue and merge request description

It allows easier text reading on big screens
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/base/variables.scss

@@ -13,6 +13,7 @@ $code_line_height: 1.5;
 $border-color: #E5E5E5;
 $background-color: #f5f5f5;
 $header-height: 50px;
+$readable-width: 1100px;
 
 
 /*

app/assets/stylesheets/pages/issuable.scss

@@ -45,3 +45,9 @@
 
   .btn { font-size: 13px; }
 }
+
+.issuable-details {
+  .description {
+    max-width: $readable-width;
+  }
+}

app/assets/stylesheets/pages/tree.scss

@@ -116,6 +116,9 @@
 }
 
 .readme-holder {
+  margin: 0 auto;
+  max-width: $readable-width;
+
   .readme-file-title {
     font-size: 14px;
     font-weight: bold;