Clean up styling of starter paragraph block/placeholder

[iamthomasbishop]

Initial "starter" block

This is in reference to the first block that appears when creating a new post – the one that has the label of "Start writing...".

Punch list

• Change font-family to Noto Serif (it looks like it is currently the system default sans-serif)
• Swap in (+) icon if possible – the unicode character feels out of place
• Make sure placeholder text color is $gray aka #87a6bc

[koke]

I've tried swapping the unicode character with the icon and it works, with some limitations:

• Since it's an inline image, it seems to be ignoring CSS, and I'm not sure if we can apply any styling to it
• The paragraph placeholder uses RichText.placeholder, which I doubt will support the attributed strings right away.