Avoid gaps in the headers and keep consistency

[lorenzo-cavazzi]

Headers (intended as <hx> components) should avoid gaps and be used sequentially.

Even after the first round of restyling (#3214), we didn't do a good enough job because we ended up skipping a few elements (mostly h1 and h3) instead of changing the size appropriately.
We should adjust that by using headers in a semantically valid way, changing the size through bootstrap variables to fit the Design team's requirements.