fix(arcgis-rest-auth): enable oAuth from within an IFrame

[dbouwman]

Adds a new check for oauth handler on window.opener, and cleans up previous handler checks.

AFFECTS PACKAGES:
@esri/arcgis-rest-auth

ISSUES CLOSED: #711

[dbouwman]

I'm going to verify this by yalc-ing this into a StoryBook and verifying the iframed oauth process works

[dbouwman]

Verified this works in scenarios where the caller is in an <iframe> in a page... it correctly (at least in chrome) locates the handler on window.opener. Unclear what's up the travis and codecov

[tomwayson]

nice work @dbouwman!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@abaaa16). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             master      #714   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?       107           
  Lines             ?      1635           
  Branches          ?       293           
==========================================
  Hits              ?      1635           
  Misses            ?         0           
  Partials          ?         0           

Impacted Files	Coverage Δ
packages/arcgis-rest-auth/src/UserSession.ts	100.00% <ø> (ø)
...es/arcgis-rest-service-admin/test/mocks/service.ts	100.00% <0.00%> (ø)
...ages/arcgis-rest-portal/test/mocks/items/search.ts	100.00% <0.00%> (ø)
...ages/arcgis-rest-portal/src/users/get-user-tags.ts	100.00% <0.00%> (ø)
packages/arcgis-rest-feature-layer/src/index.ts	100.00% <0.00%> (ø)
packages/arcgis-rest-portal/src/items/update.ts	100.00% <0.00%> (ø)
packages/arcgis-rest-auth/src/fetch-token.ts	100.00% <0.00%> (ø)
packages/arcgis-rest-portal/src/users/update.ts	100.00% <0.00%> (ø)
...kages/arcgis-rest-portal/src/users/search-users.ts	100.00% <0.00%> (ø)
...es/arcgis-rest-request/test/mocks/param-builder.ts	100.00% <0.00%> (ø)
... and 98 more

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