fix: improve UI and design based on feedback from authors

Checklist:

[x] I have read freeCodeCamp's contribution guidelines.
[x] My pull request has a descriptive title (not a vague title like Update index.md)

Closes #XXXXX

Small fixes based on feedback from Estefania:

Change the icon for embed content since the same icon was used for code block
Simplify the text in the dropdown menu to add a heading
Fix casing of the placeholder text in the title field
Make the width of the editor similar to the width of published posts
Show name field instead of Strapi's default username in the bottom left of the navigation menu

I also removed the outer <Flex> element in post-form.jsx that seemed unnecessary.

Screenshots:

freeCodeCamp / publish