A function to overwrite Popup class

[thomasconner]

Description

This PR adds a function to the User class to overwrite the Popup class used for Mobile Identity Connect. Right now, a build tool such as Webpack is used to replace the popup class used at the time a bundle is generated. This PR would eliminate the need for this.

Changes

• Add usePopupClass() function to the User class.

[codecov-io]

Codecov Report

Merging #124 into master will decrease coverage by 0.8%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- Coverage   82.06%   81.25%   -0.81%     
==========================================
  Files          78       78              
  Lines        6533     6503      -30     
  Branches     1054     1049       -5     
==========================================
- Hits         5361     5284      -77     
- Misses       1172     1219      +47

Impacted Files	Coverage Δ
src/entity/src/user.js	56.58% <33.33%> (-0.22%)	⬇️
src/datastore/src/sync.js	82.33% <0%> (-6.38%)	⬇️
src/datastore/src/cachestore.js	59.05% <0%> (-4.02%)	⬇️
src/request/src/cache.js	96.36% <0%> (-2.26%)	⬇️

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[vinaygahlawat]

LGTM