store predictions_url in runs

[amueller]

Store predictions_url from xml in runs object when getting run from server.

[mfeurer]

Thanks a lot!

[codecov-io]

Codecov Report

Merging #783 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #783      +/-   ##
===========================================
+ Coverage    87.71%   87.72%   +<.01%     
===========================================
  Files           36       36              
  Lines         4177     4187      +10     
===========================================
+ Hits          3664     3673       +9     
- Misses         513      514       +1

Impacted Files	Coverage Δ
openml/runs/functions.py	82.22% <100%> (+0.14%)	⬆️
openml/_api_calls.py	83.11% <0%> (-1.3%)	⬇️
openml/extensions/extension_interface.py	91.17% <0%> (ø)	⬆️
openml/extensions/sklearn/extension.py	91.27% <0%> (+0.03%)	⬆️
openml/flows/functions.py	87.74% <0%> (+0.32%)	⬆️

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