[FIX] website: prevent dropping snippets in <p/>

Main snippets can be dropped in `.oe_structure` elements and
`[data-oe-type=html]` elements. The second case happens when adding
a field in a page thanks to `t-field`. If used on a <p/> element,
the content cannot contain snippets (as browsers will not allow <div/>
and <section/> in a <p/>).

Note: we should not use <p/> with HTML fields at all but this might
happen with old databases... or by mistake. The problem is that the
problem is not visible during development without this fix.