Pombola/953 remove artefacts

For https://github.com/mysociety/pombola/issues/1025 and https://github.com/mysociety/pombola/issues/953

Most of changes are adding tests so that unexpected side effects would be caught.

Once merged the current contents of database will need to be changed. Will work out the SQL and add that in the comments to the tickets in Pombola.