Create new auto_generated field in Messages and stop logging patchset deletions

Create new auto_generated field in Messages. Set this field to True for Messages created for logging.

Verified in the staging datastore that auto_generated is True for the following messages:
* 'The CQ bit was (un)checked by...' from both the CQ checkbox and the 'LGTM and CQ'.
* 'xyz changed reviewers:...'

This CL also removes the patchset deletion logging as discussed in
<a href='https://groups.google.com/a/chromium.org/forum/#!msg/chromium-dev/M7f9VTqrR8w/RECOX2Y5YasJ'>https://groups.google.com/a/chromium.org/forum/#!msg/chromium-dev/M7f9VTqrR8w/RECOX2Y5YasJ</a>

[jrobbins (corp), 2014-08-27 17:57:50]

lgtm

[rmistry, 2014-08-27 18:12:35]

Submitted as
https://code.google.com/p/rietveld/source/detail?r=ab90f5e6f8a44b54026ae214ee...

[rmistry, 2014-08-27 18:15:09]

On 2014/08/27 18:12:35, rmistry wrote:
> Submitted as
>
https://code.google.com/p/rietveld/source/detail?r=ab90f5e6f8a44b54026ae214ee...

Jason, please include this in your next push.
Also, you mentioned you were waiting for 'auto_generated' messages to fix the
case where reviewers are edited in and then the publish does not contain the
diff.