Pipeline creation endpoint is not consistent with other resources

Description

Most of the endpoints use a name in its plural form for creation (POST on /issues, /merge_requests, /projects, /groups...) but the endpoint for the pipeline is singular:

• https://docs.gitlab.com/ce/api/pipelines.html#create-a-new-pipeline
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/pipelines.rb#L44

Proposal

Change the endpoint to be consistent with the other endpoints in the product, using the plural form:

POST /projects/:project_id/pipelines

Also, keep the old endpoint (but officially deprecate it) as an alias of the new one, for backward compatibility.