Allow adding a new license via the API

Drew Blessingrequested to merge
dblessing/gitlab:api_post_license into master

Fixes #313 (closed)

Add a new post license API endpoint. The endpoint is /license and the only/required attribute is license. The license should be sent in string format.

I also renamed the licenses.rb api file because it deals with license templates and was pretty ambiguous. The endpoints didn't change, and are still equally ambiguous, unfortunately. We should really consider changing those in 9.0 to /license_template or something.

cc/ @MrChrisW

@rymai Will you review, please?

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