Merge branch 'boards-filter-zindex' into 'master'

Removed z-index for filters on issue boards

## What does this MR do?

I dont know how this is has only just become a problem, but when the sidebar is open the filters on issue boards overlap it.

## Screenshots (if relevant)

### Before

![Screen_Shot_2016-11-02_at_09.50.59](/uploads/430ec15249b866999e83fe402cd9bd35/Screen_Shot_2016-11-02_at_09.50.59.png)

### After

![Screen_Shot_2016-11-02_at_09.49.40](/uploads/3a417570a874197ce4b66cbfadbf8181/Screen_Shot_2016-11-02_at_09.49.40.png)

See merge request !7235

app/assets/stylesheets/pages/boards.scss

@@ -45,11 +45,6 @@
   .page-with-sidebar {
     padding-bottom: 0;
   }
-
-  .issues-filters {
-    position: relative;
-    z-index: 999999;
-  }
 }
 
 .boards-app {