Make NewButton the same size across breakpoints

[bryanjenningz]

Closes #2343

Changes:

• Removes NewButton size changes across breakpoints so it looks more consistent and makes the button styling more simple.
• Makes NewButton easier to see and press on mobile (for example, the DOJO exercises "Solve Exercises" button was pretty small on mobile before, etc).

How to test:

• Go to the pages that use NewButton and make sure everything looks good on mobile.
  • Here are all the pages that use NewButton:
    • /exercises/js0
    • /admin/lessons/js0/introduction
    • /curriculum/js0/mentor

/exercises/js0 page on mobile (DOJO exercise list):

/exercises/js0 page on mobile (DOJO exercise card):

/exercises/js0 page on mobile (DOJO exercise card answered):

/admin/lessons/js0/introduction on mobile (this wasn't designed for mobile before this pull request so it doesn't look as good here but this can be fixed in a subsequent pull request):

/curriculum/js0/mentor on mobile (this page isn't designed for mobile so it doesn't look as good here):

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Name	Status	Preview	Updated
c0d3-app	✅ Ready (Inspect)	Visit Preview	Sep 25, 2022 at 6:17PM (UTC)

[codecov]

Codecov Report

Merging #2346 (df1d5de) into master (54bca2f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #2346   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          181       181           
  Lines         3178      3178           
  Branches       850       836   -14     
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  Hits          3178      3178