[FIX] board: avoid double-escaping in domain

This reverts commit bfadb8e4.

Since that commit, the domain may be double-encoded in the database if
the user reorganizes the layout of his dashboard: once by the JavaScript
code, and once by the serialization of the innerHTML property.

However, there is no need to have it double-encoded, and some tools such
as the upgrade scripts will fail upgrading the domain.

With this commit, we are serializing the document to XML instead of HTML
and therefore the produced output is valid XML that will be correctly
interpreted by all the tools.

However, double-encoded domains might still be present in existing
databases, so we also need to take this into account when reading the
domain.

OPW-3130117

closes odoo/odoo#119518

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>