delete webapps from the database (bug 1020465)

fixes [bug 1020465](https://bugzilla.mozilla.org/show_bug.cgi?id=1020465)

WIP, do not merge/review
Excellent point, very good catch. For reference, here's the SQL request @robhudson showed me to check we're not missing some translated fields:

```
SELECT
     k.table_name,
     k.constraint_name
FROM
     information_schema.KEY_COLUMN_USAGE k
WHERE
     k.CONSTRAINT_SCHEMA = 'zamboni' AND
     k.referenced_table_name IS NOT NULL AND
     k.referenced_table_name = 'translations'
ORDER BY
     k.table_name, k.constraint_name;
```