[FIX] web_editor: show the correct alignment in toolbar when in link

The `align` command applies the alignment to the closest block of the
selected node. Bootstrap sets the default alignment of `.btn` to
`center` but we don't support links with a width that is larger than
their contents so that alignment is not visible. For these two reasons,
we want the toolbar to show the alignment of a link's closest block
rather than that of the link itself.

closes odoo/odoo#106772

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