Added database migration for custom icons

Added database migration for the upcoming custom icons on the Button Editor.
See: https://blog.littlelink-custom.com/upcoming-features/
And: https://blog.littlelink-custom.com/progress-of-the-new-button-editor/

Author: JulianPrieber

database/migrations/2021_03_18_082008_create_links_table.php

@@ -26,6 +26,7 @@ public function up()
             $table->foreign('button_id')->references('id')->on('buttons');
             $table->timestamps();
             $table->text('custom_css')->default('');
+            $table->text('custom_icon')->default('fa-external-link');
         });
     }