Use urljoin to create thumbnail string

[fronzbot]

Description:

Uses urljoin function from requests lib to always parse thumbnails correctly.

Note: my system doesn't have the problems in the linked issue, so I cannot be certain it fixed the problem

Related issue (if applicable): home-assistant/core#67481

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 #550 (519bcec) into dev (0a03836) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #550      +/-   ##
==========================================
+ Coverage   86.23%   86.25%   +0.01%     
==========================================
  Files           8        8              
  Lines        1061     1062       +1     
==========================================
+ Hits          915      916       +1     
  Misses        146      146              

Flag	Coverage Δ
unittests	86.25% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
blinkpy/camera.py	84.49% <100.00%> (+0.08%)	⬆️

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