Add search to sitemap listing

Search functionality is added to the top of the sitemap listing in the channel manager

• 'X' appears once a user starts to type; clicking it will clear a search
• Placeholder text = Search
• The sitemap listing starts being automatically refined once a user starts typing
• A search is executed when the user presses enter

On executing a search the sitemap list updates with the relevant items and the first item in the list is given a hover-state.

*NOTE: Search bar design and functionality should be the same as implemented for https://issues.onehippo.com/browse/PM-28*