[DB upgrade] Adding a field to select the mail's body format

Technical reminder about the DB upgrade :
There doesn't seem to be an easy and reusable way to give CMail both
parts of an text+html email, so we rework the database schema to have
a simple 'body' field with a 'body_ishtml' switch, following CMail
interface.

Enhancement from issue #1

sql/update_0.3.1-0.3.2.sql

@@ -0,0 +1,21 @@
+-- Copyright (C) 2021 Chl <chl-dev@bugness.org>
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU General Public License as published by
+-- the Free Software Foundation, either version 3 of the License, or
+-- (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU General Public License for more details.
+--
+-- You should have received a copy of the GNU General Public License
+-- along with this program.  If not, see http://www.gnu.org/licenses/.
+
+
+-- We designed the schema too quickly : CMailFile doesn't let us manage
+-- independently the text and html parts. All we can do is set the mode.
+ALTER TABLE llx_sribm_custom_mail_info CHANGE COLUMN body_plaintext body mediumtext;
+ALTER TABLE llx_sribm_custom_mail_info DROP COLUMN body_html;
+ALTER TABLE llx_sribm_custom_mail_info ADD COLUMN body_ishtml smallint DEFAULT 0 NOT NULL;