Retry of duplicate slug detection

[jacqui]

This PR is an updated version of the duplicate slug detection code, testable using "latest code" in the script editor.

Articles:

• [x] preview an existing article, succeed
• [x] publish an existing article, succeed
• [x] create new doc, create new test case, set slug & category & locale to existing article values, try to preview, should get error
• [x] create new doc, create new test case, set slug & category & locale to existing article values, try to publish, should get error
• [x] click to create a new translation on existing article (franny lou -> spanish)
• [x] create new test case for new translated doc, open, open sidebar, set title & description values, try to preview & publish, succeed

Pages:

• [x] preview an existing page, succeed
• [x] publish an existing page, succeed
• [x] create new doc, create new test case, set slug & locale to existing page values, try to preview, should get error
• [x] create new doc, create new test case, set slug & locale to existing page values, try to publish, should get error
• [x] change slug in new doc, then preview, should succeed
• [x] new doc with changed slug should also publish successfully
• [x] click to create a new translation on existing page
• [x] create new test case for new translated doc, open, open sidebar, try to preview & publish, succeed