[FIX] mass_mailing: stick the snippets sidebar to its dialog header

The snippets sidebar in a dialog does not stick to the dialog title when
scrolling the dialog
- concerns: marketing_automation

How to reproduce:
- create a marketing campaign and add a new Email activity
- create a new mail template
- choose Event Promo as the Email Template
- scroll the dialog to the bottom
Issue:
- snippets sidebar scrolls alongside the iframe, resulting in parts of the
  option being hidden, instead of sticking to the dialog title.

Solution:
- unset the `overflow` css property of the `o_content` element

task-4178640

closes odoo#183602

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>
Co-authored-by: Astik Singh <assi@odoo.com>
Co-authored-by: Damien Abeloos <abd@odoo.com>
Co-authored-by: Mahdi Cheikh Rouhou <macr@odoo.com>
Co-authored-by: Shubham Thanki <shut@odoo.com>

Author: 4 people

addons/mass_mailing/static/src/scss/mass_mailing.scss

@@ -200,3 +200,7 @@ body.editor_has_snippets .o_field_mass_mailing_html div.d-flex:has(iframe) {
         min-height: 55vh;
     }
 }
+
+.o_dialog .o_content:has(.o_field_mass_mailing_html) {
+    overflow: unset;
+}