Allow defining groups of genomes in input file.

[njohner]

With this PR we introduce the possibility of defining groups of genomes.

For now the groups can only be defined in the input CSV file and they can only be used in the GWAS view.

Also note that we are no longer handling errors ourselves in the GWAS view, but let django forms take care of error handling. This has the advantage that field validation errors will get displayed on that particular field in the form, whereas other form validation errors (e.g. depending on several fields) will be displayed at the top of the form, as seen in the screenshots below. I plan on refactoring all forms in the future to apply this approach.

I will continue to work on this feature in future commits, notably to enable the use of the groups in other views, add the possibility to add and edit groups and add a group details view.

Checklist

• Changelog entry
• Check that tests still pass
• Add tests for new features and regression tests for bugfixes whenever possible.