Improve performance

This MR introduces the main file of each Tafel to be published to the assets dir (Clemens has agreed to this).

The XML can now be updated by triggering a manual job in the "Pipelines" interface of GitLab. This retrieves the data from TextGrid and a bot commits it to the repository if any changes have occurred.

To try it out, go to "Build > Pipelines", find the latest entry for this branch (see image) and click on the "play" button. Run the job by clicking on "load_data".

In this case, nothing might change since I updated the data just now, but as ed8b6ad8 illustrates, the feature works.