Generate avatar urls for users using gravatar

[nezhar]

Generates gravatar URLs for users or fallback to default if they don't use gravatar.

[codecov-commenter]

Codecov Report

Merging #86 (5fe89a0) into master (0d48d34) will decrease coverage by 0.08%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   95.10%   95.02%   -0.09%     
==========================================
  Files          62       62              
  Lines        1573     1587      +14     
==========================================
+ Hits         1496     1508      +12     
- Misses         77       79       +2     

Impacted Files	Coverage Δ
snypy/users/models/models.py	88.88% <80.00%> (-11.12%)	⬇️
snypy/users/rest/serializers.py	91.66% <83.33%> (-8.34%)	⬇️
snypy/snippets/rest/serializers.py	78.35% <100.00%> (+0.93%)	⬆️

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