Add an edit shortcut to open a post section from Customize Posts #8

Just as there is an Add New button to insert a new post and add it to a Select2 (#2) each existing selected option in a Select2 should have an edit button to open that existing post's section for editing. For a multiple select, this could be an Edit icon appearing on the left of the option, where on the right there is the X to remove the option. For non-multiple, there could be a button appearing under the Select2 control to edit the selected option.

Depends on xwp/wp-customize-posts#146 so we can fetch a post and create a section on demand.