[DevOps Adoption] Remove groups modal in favor of dropdown

Problem

As discussed in #329445 (comment 596474185) loading all groups (without the limit) in the modal leads to heavy performance issues. We need to find an alternative solution to solving the problem of only showing a limited number of groups in the modal.

Proposal

The table itself acts as this list of enabled groups on the DevOps Adoption page so we should be able to get rid of the modal and use a dropdown for adding groups to the table instead.

Let's implement the following:

• Remove the modal
• Add a dropdown above the table that allows users to add groups to the table (via ticking checkboxes)
• The dropdown shows the first 20 (?) groups and contains a search input.
• Removing groups from the table should work as usual (via the delete button in the table)