Force default timestamp for zeitreihen layer

This is for https://github.com/geoadmin/mf-geoadmin3/issues/1928

I consider this a dirty hack. We should come up with a solution for layers that don't have the last timestamp as default timestamp.

We already have the 'timeBehaviour' parameter (with `all` and `last` settings) which we could extend to include a specific timestamp to be transmitted with the layers configuration.