feat(DOJO): Add back button to addExercise page

[flacial]

Related to #2029

Description

This PR adds a back button that links to the mentor page under /curriculum/[lessonSlug]/mentor. So, once the mentor finish adding exercises, they can return to the mentor page.

Testing

1. Go to /curriculum/js0/mentor
2. Click ADD EXERCISE
3. Click the back button arrow on the top
4. Make sure you went back to the mentor page

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Name	Status	Preview	Updated
c0d3-app	✅ Ready (Inspect)	Visit Preview	Oct 3, 2022 at 11:08PM (UTC)

[codecov]

Codecov Report

Merging #2393 (d8594dc) into master (cf10813) will not change coverage.
The diff coverage is 100.00%.

❗ Current head d8594dc differs from pull request most recent head 6c30d4a. Consider uploading reports for the commit 6c30d4a to get more accurate results

@@            Coverage Diff            @@
##            master     #2393   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          184       184           
  Lines         3284      3287    +3     
  Branches       874       874           
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+ Hits          3284      3287    +3     

Impacted Files	Coverage Δ
...rriculum/[lessonSlug]/mentor/addExercise/index.tsx	100.00% <100.00%> (ø)