[16.0][FIX] base_user_locale: user can change own locale

[anusriNPS]

Made fields as part of "res.users" to be writeable: "date_format", "time_format", "week_start", "decimal_point", "thousands_sep"

base_user_locale settings works only in incognito window and verified the same

Porting from PR: https://github.com/OCA/server-ux/pull/620