fix(challenges): Implement Help Dialog

This commit applies changes from 78e86f514f.

It introduces Help dialog which guides the user to review Read-Search-Ask
methodology, ask the question on the forums or in gitter chatroom.

This dialog replaces existing Help button which was leading to Gitter Chatroom.

client/less/lib/bootstrap/modals.less

@@ -81,10 +81,6 @@
   border-bottom: 1px solid @modal-header-border-color;
   min-height: (@modal-title-padding + @modal-title-line-height);
 }
-// Close icon
-.modal-header .close {
-  margin-top: -2px;
-}
 
 // Title text within header
 .modal-title {