[FIX] mass_mailing: fix template content visibility in email

How to reproduce:
 - Create an SMS and send it
 - go to the sent option and select a contact
 - Click on add to list
 - Select a mailing list
 - then click on add and sent mailing (you will be redirected to email marketing)
 - Select a template and send it
 - The recipient will not be able to see the template (The email is blank)

Issue:
Template value is not stored in the body_html field

Technical reason:
When the record is saved the changes in `body_html` are discarded from _getchanges() https://github.com/odoo/odoo/blob/c2b160fe50c24e9f01d0d5b350f2422cfdbc72d4/addons/web/static/src/model/relational_model/record.js#L574-L581
because, field is set to readonly = 'true' after this commit https://github.com/odoo/odoo/pull/132247

After this commit:
The recipient will be able to see the template

Task-3556020

closes odoo/odoo#143293

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/static/src/js/mass_mailing_html_field.js

@@ -641,7 +641,7 @@ export const massMailingHtmlField = {
         props.iframeHtmlClass = attrs.iframeHtmlClass;
         return props;
     },
-    fieldDependencies: [{ name: 'body_html', type: 'html' }],
+    fieldDependencies: [{ name: 'body_html', type: 'html', readonly: 'false' }],
 };
 
 registry.category("fields").add("mass_mailing_html", massMailingHtmlField);