fix fluid typography and block font sizes

[madhusudhand]

This addresses the issues of fluid typography and font-sizes of few blocks.

• Add max for small and medium to avoid max being set to a factor of 1.5 by default by Gutenberg.
• Add an explicit fluid: false for large and x-large to bypass fluid calculation. Ref: Typography: add explicit bypass for fluid font size calculation gutenberg#42757
• Remove the font-size for post-title block to inherit based on the set level. h2 in single post, h4 in home-page (addresses this comment)
• Update font size of post-excerpt to match figma.

Screenshots

Home page:

Before	After

Single post title:

Before	After

Paragraphs:

Before	After

[carolinan]

With the removal of the text above the posts, the post titles on the home would be changed to h3.

[mikachan]

Thanks for this! These changes look good to me, the paragraph sizes in particular are looking really good 🎉