subgroup section

- Migration updates existing groups to "owner", then sets default to
  "maintainer" so that new groups will default to that
- Update spec examples

Both with subgroup_creation_level set to owners and to maintainers.
Also fixed the naming of some other examples in the same spec as they
were contradicting what they were actually performing in the test.
These examples were probably copy/pasted, and not renamed.

For both groups_controller and admin/groups_controller

This setting is at the group level only. The default is specified to
be maintainers and owners.

**Specs only**, all failing.

All specs passing

- Change the two existing feature examples that create a subgroup to
  elucidate that the owner is creating the subgroup

- Nest two more specs inside the 'subgroup support' context detailing
  what happens when a maintainer attempts to add a subgroup (one with
  subgroup support, and one without)

Handle trailing slashes when generating issue URLs

See merge request gitlab-org/gitlab-ce!30911