UI needs to handle all migration plugins

With #2795447: Use the core plugin manager, the drush commands can now manage any migration plugins, whether they're defined with migrate_plus's configuration entities, or the core plugins from the migrations/migration_templates folders. However, the UI still only handles configuration entities - most notably MigrationListBuilder, which extends ConfigEntityListBuilder. It would be nice if there were a PluginListBuilder in core, but apparently not...