Add group name validation for subgroups

What does this MR do and why?

This MR intends to resolve the following bug: #346883 (closed)

An attempt to create a subgroup with the same name as an existing subgroup returns a 500 error.

• Sentry(internal): https://sentry.gitlab.net/gitlab/gitlabcom/issues/3643977/events/119597597/

Logs:

ActiveRecord::RecordNotUnique
PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_namespaces_name_parent_id_type"

DETAIL:  Key (name, parent_id, type)=(SOMEGROUP, 111111, Group) already exists.
PG::UniqueViolation

ERROR:  duplicate key value violates unique constraint "index_namespaces_name_parent_id_type"

It seems like, the unique index was modified in this MR: !71390 (merged). There are no validations in the group model.

The lookup is using index. However, I'm not sure about the performance impact as it's a group level validation(conditional).

> Group.where(name: "subgroup", parent_id: 33).explain

 Index Scan using index_groups_on_parent_id_id on namespaces  (cost=0.13..2.15 rows=1 width=379)
   Index Cond: (parent_id = 33)
   Filter: ((name)::text = 'subgroup'::text)
(3 rows)

Screenshots or screen recordings

Before:

After:

How to set up and validate locally

1. Checkout this branch
2. Create a group and a subgroup
3. Create another subgroup with the same name
4. Notice that the UI shows validation errors instead of 500 error

MR acceptance checklist

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