Make addressed/unaddressed workflow opt-in

Make addressed/unaddressed workflow opt-in

The current workflow for the address/unaddressed attribute of comments
sets all comments to unaddressed by default. This is unintuitive, as it
assumes that all comments are actionable items. It also imposes a
massive burden on maintainers, who will need to manually sift through
every single comment received to a list and manually set the
non-actionable items as "addressed".

Change this workflow so that the 'addressed' field defaults to NULL.
This means maintainers or users must manually set this to False when
they're requesting additional feedback. This is currently possible via
the web UI or REST API. A future change will make it possible via a
custom mail header.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Cc: Raxel Gutierrez <raxel@google.com>
Cc: Daniel Axtens <dja@axtens.net>