Improve the lesson page spacing and elements

[flacial]

Changes

• Increase the h3 font size from 16px to 20px because when the text is short, it's hard to read it
• Add a top margin for every heading & paragraph element to make the content look less cramped
• Remove the pencil icon from the text "Edit this page" to simplify the look of it (the icon isn't compatible with the icons we use in the app)
• Add spacing between the lesson content elements: next/previous lesson, lesson text content, and challenge bar
• Redesign the ChallengeBar

Screenshots

Before & After headings:

before

after:

pencil icon

before:

after:

Challenge bar

before:

after:

Lesson content elements

before:

after:

Related PRs

#2321

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Name	Status	Preview	Comments	Updated (UTC)
c0d3-app	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 15, 2023 8:08am

[codecov]

Codecov Report

Merging #3118 (3d1dabf) into master (98917a0) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 3d1dabf differs from pull request most recent head e39b054. Consider uploading reports for the commit e39b054 to get more accurate results

@@            Coverage Diff            @@
##            master     #3118   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          191       191           
  Lines         3644      3645    +1     
  Branches       992       992           
=========================================
+ Hits          3644      3645    +1     

Impacted Files	Coverage Δ
components/EditPage/EditPage.tsx	100.00% <ø> (ø)
components/mdx/ChallengBar.tsx	100.00% <ø> (ø)
helpers/mdxComponents.tsx	100.00% <100.00%> (ø)
pages/curriculum/[lessonSlug]/[subLessonSlug].tsx	100.00% <100.00%> (ø)

[scotthallock]

Screenshots look great!