API to set namespace license for GitLab.com

Description

In order to automatically set a namespace's license when they purchase a paid plan, we need to set the license via an API.

This is required as part of the improvements to the customer checkout flow for GitLab.com https://gitlab.com/gitlab-com/customers-gitlab-com/issues/120 and specifically, is related to https://gitlab.com/gitlab-com/customers-gitlab-com/issues/124

Proposal

The API should only be able to run as an Admin of the instance and accept a namespace and a license name. This should then be set and return a success or appropriate error.

Links / references

https://gitlab.com/gitlab-com/customers-gitlab-com/issues/120 https://gitlab.com/gitlab-com/customers-gitlab-com/issues/124