tryton-env: Update display logs for new related read API

tryton-env: Update display logs for new related read API

Since issue7888, the result of related fields are stored in nested
dictionaries.

issue8426

[pokoli, 2019-07-11 14:04:59]

On tryton when showing the logs of a model that does not have write_uid nor
write_date I get the following error:

  File "/home/pokoli/projectes/nclone/tryton/tryton/gui/window/form.py", line
224, in sig_logs
    value = value.get(name, '/')

AttributeError: 'NoneType' object has no attribute 'get'

[reviewbot, 2019-07-11 14:08:03]

flake8 OK
URL: https://codereview.tryton.org/259841002

[reviewbot, 2019-07-11 14:44:19]

flake8 OK
URL: https://codereview.tryton.org/259841002

[resteve, 2019-07-12 06:02:00]

SAO: LGTM

Tryton:

name 'datetime' is not defined (could be import datetime).

[angel, 2019-07-12 08:41:49]

LGTM

[rietveld-bot_tryton.org, 2019-07-20 19:45:44]

New changeset 0e4ec5c3f8e7 by Cédric Krier in branch 'default':
Update display logs for new related read API
https://hg.tryton.org/sao/rev/0e4ec5c3f8e7

[rietveld-bot_tryton.org, 2019-07-20 19:45:46]

New changeset 450b5322f9ee by Cédric Krier in branch 'default':
Update display logs for new related read API
https://hg.tryton.org/tryton/rev/450b5322f9ee

[rietveld-bot_tryton.org, 2019-07-20 19:45:49]

New changeset fd6d7a271c97 by Cédric Krier in branch 'default':
Update display logs for new related read API
https://hg.tryton.org/tryton-env/rev/fd6d7a271c97

[rietveld-bot_tryton.org, 2019-07-26 19:16:50]

New changeset fe7503b3f7a1 by Cédric Krier in branch '5.2':
Update display logs for new related read API
https://hg.tryton.org/sao/rev/fe7503b3f7a1

[rietveld-bot_tryton.org, 2019-07-26 19:16:51]

New changeset cc04ece478f1 by Cédric Krier in branch '5.2':
Update display logs for new related read API
https://hg.tryton.org/tryton/rev/cc04ece478f1