Run a migration group from UI [#2996610]

Problem/Motivation

From CLI, running a migration group is easy: drush mim --group=foo.
It would be great to also have a way to do that easily from the UI.

Proposed resolution

Expose a new operation "Execute" operation for each of the listed migratioin groups at /admin/structure/migrate.
Then a similar UI than the presented on each migration run, like the migrate_tools.execute route, will be presented.
The execution will happen in a batch operation.

Reviewing a bit how the import is working on the current UI, I see the \Drupal\migrate_tools\MigrateBatchExecutable class instantiated and called from the form submit handler. But since it extends Drupal\migrate_tools\MigrateExecutable, which in turn extends Drupal\migrate\MigrateExecutable, only one migration is assumed to be passed, e.g. the following relevant line in \Drupal\migrate_tools\MigrateBatchExecutable::batchImport():

    $operations = $this->batchOperations([$this->migration], 'import', [
      'limit' => $this->itemLimit,
      'update' => $this->updateExistingRows,
      'force' => $this->checkDependencies
    ]);

It may be a good idea to change how this works to be able to pass several migrations, or even a migration group.

It seems like #2921721: Consolidate CLI functionality into a service may be a a way to generalize part of the shared functionality, and may allow re-using a service from both drush and UI, so maybe that issue could block this one.

Work-around: It is always possible to run all the migrations in the group manually one by one, but that can become tedious if the number of them is not small.