Follow linked-from to get id in image fetching (fixes #171)

[adamcik]

No description provided.

[codecov]

Codecov Report

Merging #245 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   96.76%   96.76%   +<.01%     
==========================================
  Files          13       13              
  Lines        1174     1176       +2     
==========================================
+ Hits         1136     1138       +2     
  Misses         38       38

Impacted Files	Coverage Δ
mopidy_spotify/images.py	100% <100%> (ø)	⬆️

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[kingosticks]

LGTM once black passes