[FIX] mail: mail template in Discuss

When the mail composer is opened thanks to the "Send Mail" button in
Discuss, it's possible to save a mail template, but the user can't
access it anymore later on.

This is because the field `model` is by default empty in this specific
case since this is a client action. However, when saving the template,
the default value 'mail.template' is used for the `model` field.
Therefore the domain on the `template_id` field prevents the user to
find back the model he just saved.

The fix sends the `active_model` in the context with the value
'mail.message', so that the `default_get` method of
`mail.compose.message` will set the appropriate default value.

opw-677936