Add exercise preview cards to DOJO exercises page

[bryanjenningz]

This pull request adds a new ExercisePreviewCard component which gets used on the DOJO exercises page.
The exercise preview cards are populated with mock data on the frontend, but this will be removed once the backend returns DOJO exercise preview data.

This change doesn't affect any user-facing pages since there are no links to the /exercises/:lessonSlug page right now.

How to test:

• Go to /exercises/js0
• Make sure everything looks good

This pull request is a part of #2253.

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Name	Status	Preview	Updated
c0d3-app	✅ Ready (Inspect)	Visit Preview	Sep 12, 2022 at 4:19AM (UTC)

[codecov]

Codecov Report

Merging #2281 (ee742c6) into master (7563379) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2281   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          177       178    +1     
  Lines         3066      3077   +11     
  Branches       818       823    +5     
=========================================
+ Hits          3066      3077   +11     

Impacted Files	Coverage Δ
...onents/ExercisePreviewCard/ExercisePreviewCard.tsx	100.00% <100.00%> (ø)
pages/exercises/[lessonSlug].tsx	100.00% <100.00%> (ø)

[SlyBouhafs]

The answered state badge looks a bit big, can we make it the same size as the admin badge?

[bryanjenningz]

@SlyBouhafs Thanks for suggestion. I updated the badge size.

[flacial]

We decided to create a folder for each component. This folder will contain the component itself, its styles, and test file.

When creating new components, refer to #2080

[bryanjenningz]

@flacial Thanks for letting me know. I just changed the code so that the component file and styles file are in the same folder. I tried putting the storybook file in the same folder too but it didn't show up when I ran storybook, so I moved the storybook file back to the storybook folder with all the other storybook files.