[MERGE] base: remove field source of ir.translation

Instead, make sure the field src is always up to date

Change a bit the behaviour of `_write` to always try to update potential
translations when in monolanguage (was only done in multi-language if different
than en_US) and always create translations when writing on a translatable field
when in multi-language (was skipped in en_US)

Add tests

Task-id: 2031752
Pad: https://pad.odoo.com/p/r.dbf835d0aae979d858f73446dcbfb1df



closes odoo/odoo#34598

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>