[FW][FIX] web_editor: fix page scroll after scrolling a modal

[fw-bot]

Steps to reproduce the bug:

• In "Website" edit mode.
• Drag and drop enough snippets onto the page to make the vertical
  scrollbar appear.
• Drag and drop the "Popup" snippet onto the page.
• Drag and drop enough snippets into the "Popup" to make the vertical
  scrollbar appear.
• Close the popup.
• Drag a snippet from the snippet menu without releasing the mouse.
• Move it to the bottom of the page.
• Bug: the page doesn't scroll.

In the following commit 1, we enabled "smooth scroll on drag" to work
within a modal. The element being scrolled was indeed the modal when
dragging a snippet over it. However, in a later commit 2, the behavior
was changed and caused the bug. After closing the modal, we did not
redefine the scrollable element, so it remained the modal instead of the
page.

task-3854032

Forward-Port-Of: #162166

[robodoo]

Pull request status dashboard.

[fw-bot]

@bvr-odoo @SergeBayet cherrypicking of pull request #162166 failed.

stdout:

Auto-merging addons/web_editor/static/src/js/editor/snippets.editor.js
CONFLICT (content): Merge conflict in addons/web_editor/static/src/js/editor/snippets.editor.js

stderr:

19:17:15.921016 git.c:463               trace: built-in: git cherry-pick 103c9d627e1760ebc8e8d4bb8659531246cb563b
error: could not apply 103c9d627e17... [FIX] web_editor: fix page scroll after scrolling a modal
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

⚠️ after resolving this conflict, you will need to merge it via @robodoo.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[bvr-odoo]

@SergeBayet The bug doesn't exist in 17.0 thanks to this commit