Refactor login logic in preperation for 2FA

[fronzbot]

Description:

Moves login logic to separate module for easier maintenance in prep for 2FA support

Checklist:

• Local tests with tox run successfully PR cannot be meged unless tests pass
• Changes tested locally to ensure platform still works as intended
• Tests added to verify new code works

[codecov]

Codecov Report

Merging #241 into dev will decrease coverage by 1.65%.
The diff coverage is 86.74%.

@@            Coverage Diff             @@
##              dev     #241      +/-   ##
==========================================
- Coverage   82.78%   81.13%   -1.66%     
==========================================
  Files           7        8       +1     
  Lines         674      689      +15     
==========================================
+ Hits          558      559       +1     
- Misses        116      130      +14     

Flag	Coverage Δ
#unittests	81.13% <86.74%> (-1.66%)	⬇️

Impacted Files	Coverage Δ
blinkpy/api.py	68.23% <50.00%> (-8.60%)	⬇️
blinkpy/blinkpy.py	80.47% <76.92%> (-1.35%)	⬇️
blinkpy/login_handler.py	89.83% <89.83%> (ø)
blinkpy/helpers/constants.py	100.00% <100.00%> (ø)
blinkpy/helpers/util.py	88.67% <100.00%> (-6.42%)	⬇️

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