Added capability to merge organisations

- Added an interface for organisation
- Added beforeDelete and beforeMerge callbacks
- Added hard/soft delete to organisations

src/Database/Seeds/OrganisationPermissions.php

@@ -61,6 +61,12 @@ class OrganisationPermissions extends BaseSeed
                 'conditions'  => 'always()',
                 'description' => 'Edit basic properties of any organisation.',
             ]),
+            'merge_organisations' => new Permission([
+                'slug'        => 'merge_organisations',
+                'name'        => 'Merge two organisations',
+                'conditions'  => 'always()',
+                'description' => 'Merge two organisations together, including all the members.',
+            ]),
             'delete_organisation' => new Permission([
                 'slug'        => 'delete_organisation',
                 'name'        => 'Delete organisation',