User creates user groups using web service `api/user_groups/create` in a reaction to an event rather than from UI.
In some cases same events may come twice in a very short time and two simultaneous calls to API are done, which produce duplicates in a GROUPS table.
For history reasons, when MySQL was still supported, there was a limitation which prevented to set a unique constraint on column >255. Starting from 7.9 that's not the case.
We can now add a constraint with proper data migration.