[IMP] website: review structure snippets

This commit reviews a bunch structure snippets to streamline their use
and edition, namely:

Banner, Cover, Picture and Text-block snippets:
- Remove extraneous levels of edition (buttons, heading/text only
columns)

Picture snippet:
- make picture bigger

Three columns and comparison snippets:
- Make regular column options available on the cards
- Remove unwanted UI elements (card footer, button)

Cover snippet:
- Make title bold

It also consolidates the main colorpicker and the column colorpicker
into a single option as they did the same thing and allows inner content
to be dropped next to images in the Image - Text and Text - Image
snippets.

Part of https://github.com/odoo/odoo/pull/42937
task-2088157
task-2152184

addons/website/static/src/scss/website.scss

@@ -380,6 +380,11 @@ table.table_desc tr td {
     @extend .o_half_screen_height;
 }
 
+// Allows custom border radius without contents overflowing.
+.card {
+    overflow: hidden;
+}
+
 //
 // Snippets
 //

addons/website/static/src/snippets/s_three_columns/000.scss

@@ -1,5 +1,5 @@
-.s_three_columns {
-    .align-items-stretch .card {
+.s_three_columns:not([data-vcss]) {
+    .align-items-stretch > .card {
         height: 100%;
     }
 }

addons/website/static/src/snippets/s_title/000.scss

@@ -1,5 +1,5 @@
 
-.s_title {
+.s_title:not([data-vcss]) {
     .s_title_boxed {
         > * {
             display: inline-block;