Replace querystring implementation with qs module

[selaux]

Rationale: Writing a proper querystring parse implementation can be hard, so we should leverage existing modules to be more robust. Also there is an issue with the current implementation described in auth0/lock#913.

Scope: Replaces all occurences of helpers/qs.js with the qs module

Notes

• I removed this test, because it was actually testing the querystring implementation
• I had to adapt this test as pluses in urls are decoded as spaces, the new expected value seems more correct to me, but correct me if i am wrong

[luisrudge]

Can you share the bundle size difference with this module as a dependency?

[codecov-io]

Codecov Report

Merging #382 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #382      +/-   ##
==========================================
- Coverage   97.54%   97.51%   -0.03%     
==========================================
  Files          35       34       -1     
  Lines        1179     1166      -13     
  Branches      198      195       -3     
==========================================
- Hits         1150     1137      -13     
  Misses         29       29

Impacted Files	Coverage Δ
src/authentication/passwordless-authentication.js	100% <100%> (ø)	✅
plugins/cordova/popup-handler.js	91.89% <100%> (ø)	✅
src/web-auth/index.js	97.9% <100%> (ø)	✅
src/helper/popup-handler.js	100% <100%> (ø)	✅
src/authentication/index.js	100% <100%> (ø)	✅

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