[FIX] mass_mailing: ensure proper padding around plain text mailing

The plain-text mailing template had no top padding, which made it
awkward to edit and tended to make users manually add space at the top
of the mailing with paragraph breaks. This adds the same padding to the
mailing as is applied to the editor in the Note module.

Note: since the mailing is plain-text, that padding shouldn't and isn't
saved in the mail to send, resulting in a visible difference between the
Mail Body and Mail Debug tabs. This is normal.

task-2760178

closes odoo/odoo#86043

X-original-commit: 82d40ab312fc506c6e248ef3c01cccc99ce6ff73
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/scss/themes/theme_basic.scss

@@ -35,10 +35,8 @@
 .o_basic_theme {
     &.o_layout {
         margin: 0;
-        padding: 0;
+        padding: 24px 16px 10px;
         background-color: white !important;
-        padding-left: 1rem; 
-        padding-right: 1rem;
 
         &, p, h1, h2, h3, h4, h5, h6, span, ul, ol {
             color: black;