Change: Label for OcSelect

We've added a configurable `<label>` for OcSelect accessible via the `label` property.
This shadows the `label` property of `vue-select`. Hence we introduced the `optionLabel`
property on OcSelect which maps to the `label` property of `vue-select`.

https://github.com/owncloud/owncloud-design-system/pull/1633
https://github.com/owncloud/owncloud-design-system/pull/1570
https://github.com/owncloud/owncloud-design-system/issues/1503
