Convert blog post titles to h1 while keeping the styles

[bhumi1102]

This PR addresses this SEO task

• Update the blog post tiles and "category" headings while are displayed in the same location on /blog
• Update styles to maintain current font size and weight, otherwise the blog post titles look too huge (screenshot attached).

[bhumi1102]

CC: One more SEO task @AmandaPerino, @jathayde

[bhumi1102]

Without css update, the titles look like this:

before:

after:

[jathayde]

@bhumi1102 looking at the code we've got a few things to namespace to make this work I feel. So the headlines are defined as .common-headline hn where n is the number 1-6 and then there's a mobile-first responsive style for min-width: 1024px. This is defined in _sass/common/headline.scss.

For these particular pages, the div is classed with .post so we could say .post h1 and then copy in the H2 items either on headline.scss or post.scss (I feel the latter makes more sense.

How do you feel about that?