[FIX] mass_mailing: properly set new background colors in columns

Colorpicker snippets target <td> elements while these sometimes contain
a <div> which is background-colored in css. In consequence, when using
the colorpicker to change the background color of the snippet, the
colored <div> came in front of the element of which we tried to change
the color. This ensures the snippets target the <div> instead when the
<td> itself is marked as non-colorable via the "o_mail_no_colorpicker"
class, as this is then in fact the element we want to color.
Enterprise themes need to be adapted to this new behavior.

Author: Zynton

addons/mass_mailing/views/snippets_themes.xml

@@ -576,7 +576,7 @@
                         </td>
                     </tr>
                     <tr>
-                        <td>
+                        <td class="o_mail_no_colorpicker">
                             <div class="bg-o-color-2">
                                 &amp;nbsp;
                             </div>
@@ -865,7 +865,7 @@
         data-selector=".note-editable > div:not(.o_layout), .note-editable .oe_structure > div, td, th"
         data-exclude=".o_mail_no_resize, .o_mail_no_options"/>
 
-    <div data-selector=".note-editable > div:not(.o_layout), .note-editable .oe_structure > div, td, th"
+    <div data-selector=".note-editable > div:not(.o_layout), .note-editable .oe_structure > div, td, td.o_mail_no_colorpicker div:first-child, th"
          data-exclude=".o_mail_no_colorpicker, .o_mail_no_options">
         <we-colorpicker string="Background Color"
             data-select-style="true"