Update header to header spec

[muffinresearch]

Fixes: mozilla/addons#9589

Before:

After:

[coveralls]

Coverage remained the same at 100.0% when pulling 0b846b1 on muffinresearch:update-heading-css-to-spec into 70015f0 on mozilla:master.

[mstriemer]

I think the heading text looks nicer at 22px, the copy is definitely better though. Maybe we can leave the heading at 22px and @pwalm won't notice?

r+

[muffinresearch]

I think let's do it to spec and ask UX if they want any changes when they look at the real thing in practice.

Fwiw: re: the h1 the headings in the mocks looked to have letter-spacing which I'd previously eye-balled so it's not just the font-size change here the letter-spacing is also removed back to the default.

[mstriemer]

Ah, yeah that's what it is. It looks cramped to me now.

[pwalm]

Let's stick with "After." The type looks more balanced. 👍