models: add addressed field

models: add addressed field

Currently, there is no state or status associated with comments. In
particular, knowing whether a comment on a patch or cover letter is
addressed or not is useful for transparency and accountability in the
patch review and contribution process. This patch is backend setup for
tracking the state of patch and cover comments.

Add `addressed` boolean field to patch and cover comments to be able to
distinguish between unaddressed and addressed comments in the
patch-detail page.

Signed-off-by: Raxel Gutierrez <raxel@google.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
[dja: give the migration a more meaningful name]
Signed-off-by: Daniel Axtens <dja@axtens.net>