Commit [1] added a margin-bottom on buttons within snippets to make some
breathing space when several buttons follow each other and appear on
multiple lines on mobile.
However, buttons added by the customer do not have a margin, which
causes misalignment when they add it next to a prebuilt button.
This commit checks that whenever a user adds a new button, if any of its
direct siblings is a button, the newly added button and its button
siblings get an appropriate margin-bottom.
It also reverts commit [1] and removes the `mb-2` class on the button in
`s_text_cover.xml` from commit [2], as it is not needed anymore with
this new behavior.
task-3369604
[1]: https://github.com/odoo/odoo/commit/075e915
[2]: https://github.com/odoo/odoo/commit/5ac340bclosesodoo/odoo#125824
Related: odoo/design-themes#680
Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>