Add dropdown-menu-right class to the repo-watching dropdown

The dropdown menu was dropping down to the right and appearing outside of the page container. This made it not visible at smaller browser widths.

This commit makes it drop down into the page container, instead of out, and keeps it visible at all screen sizes.

templates/repo/nav.tmpl

@@ -39,7 +39,7 @@
                     <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">
                         <span class="caret"></span>
                     </button>
-                    <div class="dropdown-menu">
+                    <div class="dropdown-menu dropdown-menu-right">
                         <div class="dropdown-item text-left to-unwatch">
                             <h4 role="presentation" class="dropdown-header {{if not .IsRepositoryWatching}}text-primary{{end}}">Not Watching</h4>
                             <p class="description">You only receive notifications for conversations in which you participate or are @mentioned.</p>