[FIX] mail: plain-text messages word-wrap in Discuss UI

When a user sends a plaintext email to a mail-enabled channel, the
rendering of this message in Discuss have present a horizontal scroll
when the text contains long sentences.

This commit fixes it by enabling wrapping of the line's word to fit its
container size and prevent horizontal scrolling.

Note: the required CSS rules were already present since odoo/odoo@0e7e18ff71
but a wrapper element (with the class `.o_thread_message_content`) was
introduced in odoo/odoo@4685e4bc7f and
merged between the testing of the fix above and its actual merging in the
Odoo codebase, making it ineffective. We only have to properly re-enable
it.

opw-2341485

Related task ID: 34931

closes odoo/odoo#59719

X-original-commit: 5e66ca8d2c970eed999782bdd59e9082f3967833
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/mail/static/src/components/message/message.scss

@@ -48,6 +48,11 @@
     // the style without this rule.
     overflow: auto;
 
+    > pre {
+        white-space: pre-wrap;
+        word-break: break-word;
+    }
+
     > p:last-of-type {
         margin-bottom: 0;
     }

addons/mail/static/src/scss/thread.scss

@@ -84,10 +84,9 @@
             min-width: 0;
             max-width: 100%;
             word-wrap: break-word;
-            > pre {
+            .o_thread_message_content > pre {
                 white-space: pre-wrap;
                 word-break: break-word;
-                text-align: justify;
             }
 
             .o_mail_note_title {