Clients committing values where nothing changed such as for example
memory: 8912 => 8192 have created and entry in the history.

The adminapi client was nice enough to filter these out but others such
as the Powershell one or the Puppet (Ruby) one not.

We should not rely on the client to send us valid data even if this is
not security relevant. There is now a check in place to check for noop
changes and filter them out server side.

Add a Django command to clean up the no-op changes caused before this
fix. They provide no value to the history and can safely be dropped.

A fix preveting with the no-op changes caused by type mismatch
e.g. memory: "8" => 8 (string, integer) will follow.

Clients may send numbers and booleans as strings. The committer passed
them on untyped, so memory: 8192 => "8192" was treated as a change,
logged with the string in the changelog and, for booleans, the string
"false" was stored as true because the boolean model only checks
truthiness. This is the source of 2.17 million bogus changelog entries
that the no-op filter could not catch.

Cast the values of update, new and multi changes to the attribute type
right after validation, before the no-op filter, storage and logging.
Only number and boolean attributes need it, other types are stored from
their string form anyway. Invalid strings now fail early with a
ValidationError naming the attribute instead of in the model's
full_clean.

Clearing an unset attribute in the servershell sends a delete action
with old = null. Validation accepts it because the stored value is null
as well, nothing gets deleted, but a ChangeCommit and a Change row were
still written.