Hide signup/signin pages from authenticated user, redirect to / if accessed

[Brikaa]

I didn't use useEffect() here since I didn't find a need to render the sign-in/sign-up page before the check. Returning null will not render anything.

[Brikaa]

We could consider redirecting to /projects if an authorized user accesses /

[codecov[bot]]

Codecov Report

Merging #343 (845c9f5) into master (2cac0fd) will increase coverage by 0.33%. The diff coverage is 20.00%.

:exclamation: Current head 845c9f5 differs from pull request most recent head fe24578. Consider uploading reports for the commit fe24578 to get more accurate results

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   60.87%   61.21%   +0.33%     
==========================================
  Files         126      126              
  Lines        1958     2202     +244     
  Branches      380      534     +154     
==========================================
+ Hits         1192     1348     +156     
- Misses        731      848     +117     
+ Partials       35        6      -29     

Impacted Files	Coverage Δ
src/helpers/user-auth-helper.ts	56.25% <0.00%> (-29.47%)	:arrow_down:
src/pages/signin.tsx	0.00% <0.00%> (ø)
src/pages/signup.tsx	0.00% <0.00%> (ø)
src/components/index-page/hero.tsx	95.23% <75.00%> (-4.77%)	:arrow_down:
src/components/shared/buttons/project-button.tsx	20.00% <0.00%> (-13.34%)	:arrow_down:
src/components/shared/page/page-body.tsx	87.50% <0.00%> (-12.50%)	:arrow_down:
src/components/account/account-settings.tsx	78.94% <0.00%> (-7.72%)	:arrow_down:
src/components/shared/form/controls2.tsx	36.84% <0.00%> (-6.91%)	:arrow_down:
src/components/shared/social-icon.tsx	26.66% <0.00%> (-6.67%)	:arrow_down:
src/components/shared/seo.tsx	89.47% <0.00%> (-4.28%)	:arrow_down:
... and 55 more

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