Add views to add, delete, and display groups.

[njohner]

In this PR we add all necessary views to work with groups directly in the webapp:

• A groups overview view, displaying a list of all defined groups
• A detailed view for each group, showing the list of genomes in the group
• A view to add new groups
• A view to delete existing groups.

I did not bother to add a view to edit existing groups, as it seems easy enough to delete an existing group and create a new one instead. If need be we can always add that later.

Here a few screenshots:
groups overview

group detail view

group add form

Checklist

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