Customise slug base name for different models

fixes #2480

- when the santisation of a slug results in an empty string, we use a basename instead.
- this was 'post' and is now the table name for the model, minus the trailing 's'
- this isn't massively robust but works as a temporary fix
Merge pull request #2484 from ErisDS/issue-2480

Customise slug base name for different models