Wrong heading levels on main page (Accessibility)

The top page at www.torproject.org uses the wrong heading levels. Using the right heading levels helps structure the page for screen reader users.

1. For the "Donate Now" button at the top and footer of the page see #336 (closed)
2. The top heading "Browse Privately. Explore Freely." is split into two separate <h1> elements, they should both be contained within the same <h1>.
3. The sub-heading "Defend yourself against tracking and surveillance. Circumvent censorship." under is inside a <h3>. Sub-headings of the same heading should just be inside a <p> element. You could wrap it in a <hgroup> element who's most recent content model matches this (https://html.spec.whatwg.org/multipage/sections.html#the-hgroup-element) but it is not as important because this content model is quite recent and has no accessibility mappings right now (https://www.w3.org/TR/html-aria/#el-hgroup).
4. The "BLOCK TRACKERS", "DEFEND AGAINST SURVEILLANCE", etc headings are in <h4> rather than <h2>.
5. The "ABOUT US" heading is in an <h6> rather than <h2> in its own <section>.
6. In the footer "Our mission:" is in an <h5> rather than <h2>.
7. For the social media icons see #337
8. The "Subscribe to our Newsletter" should probably be inside a <h2> to be consistent.

Note to easily test the above, if you are using Orca screen reader (can switch on on GNOME with Super+Alt+S, you have to switch it on before opening firefox or tor browser), focus the web page and press Alt+Shift+H and you get a popup with all the headings and their levels.