Make sluggification visible, customizable, and fail-safe

Default sluggification is simplistic, relying on a title field and performing no inference whatsoever.

**With no field named "title", entries currently fail to save.**

The ideal fix is to simply make slugs editable in the editor UI.

Considerations:

* Slug editing can be switched off through config
* If slug editing is switched off, each collection config must be validated on CMS load to ensure a slug can be derived
* Prepopulate with the same best guess or configured slug template result that we currently use
* Only valid characters may be entered in the slug field
* Slugs and filenames are not always the same, so it should be possible to provide them separately somehow (see [comment](https://github.com/netlify/netlify-cms/issues/445#issuecomment-377425752) below for example)