Sign out and i18n

[powerful23]

Issue #, if available:
to sign out user correctly, move i18n to common and add unit tests for i18n
Description of changes:
When only using auth, the credentials_source will be null once reloaded which causes the user.signOut() not called

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #700 into master will decrease coverage by 0.22%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
- Coverage   87.28%   87.06%   -0.23%     
==========================================
  Files          72       74       +2     
  Lines        3429     3494      +65     
  Branches      652      668      +16     
==========================================
+ Hits         2993     3042      +49     
- Misses        419      435      +16     
  Partials       17       17

Impacted Files	Coverage Δ
packages/aws-amplify/src/Common/I18n/I18n.ts	94.44% <100%> (ø)
packages/aws-amplify/src/Common/I18n/index.ts	43.75% <100%> (ø)
packages/aws-amplify/src/Common/index.ts	81.81% <100%> (+0.86%)	⬆️
packages/aws-amplify/src/Auth/Auth.ts	88.66% <90%> (+0.57%)	⬆️

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[richardzcode]

👍

[github-actions]

This pull request has been automatically locked since there hasn't been any recent activity after it was closed. Please open a new issue for related bugs.

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