Add a sql command to roundup-admin to allow running sql commands against the database.

Specific use case is to toggle sqlite db from delete to wal journal mode and back.
The admin/upgrading documentation uses sqlite3. If sqlite3 is not installed
(e.g in a docker container) Roundup should provide some alternate method.

If the sql command is run against a non-sql backend it should return an appropriate
error.

Open question:

  should the sql command allow arbitrary sql, or should it be filtered (e.g. to rule
  out dropping tables)?