Refactor slugs

[paulrobertlloyd]

Argh. Slugs are a pain. They’re an essential component of the Micropub endpoint and need to be stable and predictable as they can affect generated paths (if a path don’t exist in a content store, things quickly fall apart).

This PR:

• reverts aspects of Decouple slug property from slug path tokens; add random path token #719, such that a slug property is always created and provided to the postTemplate; either from the value provided in mp-slug, a slugified name property, else a random 5-character string.
• fixes the slug property not being remembered when a post is deleted
• updates the post form to read from an existing slug property when editing a post
• removes the {random} path token, as this token gets regenerated each time the renderPath function is called
• removes the {uuid} path token for the same reasons
• removes slug property from front matter when using the Eleventy present (Eleventy supplies page.fileSlug)
• removes slug property from front matter when using the Jekyll present
• removes slug property from front matter when using the Hugo present (with Hugo slug is a significant property that affects the published URL, which may lead to unintended URLs if the slug not being used in the post file path)

Need more use cases for it makes sense to supply slug property to front matter properties for different static site generators, however if using postTemplate, the slug property will be provided.

/cc @aciccarello