Restoring swagger

On commit 0ef1ae7ffa3b5762e3a8e9581d4bdf0907d6c900, swagger was inadvertly
removed due to documentation removal. The following commit restores swagger-ui:

Now, the specs are available at frontend/api/modules/docs/specs (inside the doc
module) and the swagger-ui is located in web/swagger-ui/ (where the statically
generated web site is stored)