Default value needed for region if region information is missing

[LDSamson]

If no region is available, the region should default to something like other or Unknown. It is now not possible to select sites to review in mod_review_config if a region is not set:

Currently, regions are only added in apply_study_specific_fixes which is not ideal.

[LDSamson]

One way to generalize regions/ make them customizable:

We could add a new variable region_code in column_names in the metadata.

Then we can add somewhere in the metadata information about the regions, which we can use to create the region variable using a new add_regions() helper function .