Roles management

### Describe the problem

Currently, the roles can be only defined in Django admin (/admin/weblate_auth/role/) which we don't want people to use (https://github.com/WeblateOrg/weblate/issues/2363). This functionality should be exposed in Weblate management interface as well.

### Describe the solution you would like

* Add *Roles* next to *Teams* in /manage/
* The editing should not be possible for built-in roles, as these are discarded upon update (see https://docs.weblate.org/en/latest/admin/access.html#default-teams-and-roles).

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

https://github.com/orgs/WeblateOrg/discussions/14293