add columns field to revision, remove index_column

database/migrations/2018_07_08_193600_create_revisions_table.php

@@ -19,8 +19,9 @@ class CreateRevisionsTable extends Migration
             $table->unsignedInteger('refs'); // used for reference counting
             $table->unsignedInteger('ancestor_id')->index()->nullable(); // parent revision
 
-            // a column that can be used as the primary key for a table; possibly a composite column, if using a comma
-            $table->string('index_column');
+            // columns specification
+            // array with: {name, title, type}
+            $table->jsonb('columns');
 
             // author's note describing what has been changed
             $table->text('note');