[FIX] mail: preprocess tracking fields of type date/datetime only once

Before this commit, if a message thread was loaded 2 times, the first time
the date was well formatted, the second time that fail because momentjs cannot
parse the localized date from some locale.

Luckily, it was not bugged by default in english, because moment was able to
reparse a localized US date. But in some local as french, momentjs cannot parse
the localized date and return 'Invalid date'

Now, we move the code, to be processed only once by the chat manager, and not
in each process of the thread.

This commit closes issue #13258

Courtesy of @aab-odoo