Resolves the backend half of
https://gitlab.com/gitlab-org/gitlab/-/issues/22794
by changing the POST to an ajax request. This means that a reload of the
page will no longer trigger the creation of a new deploy token.

These changes are behind a feature flag named `:ajax_new_deploy_token`.
The flag exists at the group and project level.

When enabled:

- Makes create_deploy_token actions of ci_cd controllers ajax
- The JSON response matches the existing API schema

API Schema: https://docs.gitlab.com/ee/api/deploy_tokens.html