Add "create release" endpoint to api.js

Nathan Friendrequested to merge
nfriend-add-create-release-api-wrapper into master

What does this MR do?

Updates our JavaScript API wrapper (api.js) to include a method that creates a new release (POST /projects/:id/releases).

In addition, this MR adds some tests that were missing for other release-related endpoints in this file.

Edited by Nathan Friend

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