Allow adding a new license via the API

Fixes #313 

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?

See merge request !400