[FIX] mass_mailing: keep the cursor in the promo code when emptying it

When selecting the contents of a promo code then typing a letter, that
letter was inserted before the promo code instead of in it. It was also
impossible to get back inside the promo code after that.

Given the nature of that snippet it seems reasonable to make the promo
code itself unremovable. This way we can benefit from the editor's
ability to keep the cursor in an empty unremovable inline.

Note: This fix requires a change in the editor:
https://github.com/odoo-dev/odoo-editor/commit/1c98f141458de297a4b819f3fb21f532986e9872.
Do not merge without it.

closes odoo/odoo#70930

X-original-commit: ffe0277613f73e046c4e343cfb0197c937063717
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/views/snippets_themes.xml

@@ -609,7 +609,7 @@
                                 Use This Promo Code BEFORE 1st of August
                             </p>
                             <p class="o_mail_h_padding text-center">
-                                <span style="line-height: 30px;"><small>CODE: </small></span><strong class="o_code h3">45A9E77DGW8455</strong>
+                                <span style="line-height: 30px;"><small>CODE: </small></span><strong class="o_code h3 oe_unremovable">45A9E77DGW8455</strong>
                             </p>
                             <p class="text-center">
                                 and save $20 on your next order!