Continue implementation of the license template selector and /licenses API endpoint

CHANGELOG

app/assets/javascripts/api.js.coffee

app/assets/javascripts/blob/blob_license_selector.js.coffee

app/assets/javascripts/blob/edit_blob.js.coffee

app/assets/javascripts/blob/new_blob.js.coffee

app/controllers/projects/blob_controller.rb

app/helpers/blob_helper.rb

app/helpers/projects_helper.rb

app/models/repository.rb

app/views/projects/_readme.html.haml

app/views/projects/blob/_editor.html.haml

app/views/projects/blob/new.html.haml

app/views/projects/empty.html.haml

app/views/projects/show.html.haml

doc/api/README.md

doc/api/licenses.md

lib/api/entities.rb

lib/api/licenses.rb

...projects/files/project_owner_creates_license_file_spec.rb

...sees_link_to_create_license_file_in_empty_project_spec.rb