Update API endpoints for the Approval Rules frontend app

Proposal

Update the API endpoints in Approval Rules frontend app.

GET https://gitlab.com/api/v4/projects/278964/approval_settings -> GET https://gitlab.com/api/v4/projects/278964/approval_rules
POST /api/v4/projects/<project_id>/approval_settings/rules -> POST /api/v4/projects/<project_id>/approval_rules
PUT /api/v4/projects/<project_id>/approval_settings/rules/:id -> PUT /api/v4/projects/<project_id>/approval_rules/:id
DELETE /api/v4/projects/<project_id>/approval_settings/rules/:id -> DELETE /api/v4/projects/<project_id>/approval_rules/:id 

Slight differences in the response, courtesy of @vyaklushin:

https://gitlab.com/api/v4/projects/278964/approval_settings	https://gitlab.com/api/v4/projects/278964/approval_rules