Minor post OHBM enhancements

[adelavega]

• Show N subjects instead of N runs for tasks. Closes Show number of subjects for dataset listing #418
• Show contrast name warning at the bottom, and clear if fixes. Closes Show warning at the bottom #591
• Open NV links in new window. Closes Open new window for external (NV) links #604
• Fix missing pliers citation. Closes Pliers citation missing #598

[codecov-io]

Codecov Report

Merging #606 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #606      +/-   ##
==========================================
+ Coverage   72.38%   72.39%   +0.01%     
==========================================
  Files          53       53              
  Lines        2053     2054       +1     
==========================================
+ Hits         1486     1487       +1     
  Misses        567      567

Impacted Files	Coverage Δ
neuroscout/resources/analysis/endpoints.py	90.84% <ø> (ø)	⬆️
neuroscout/models/dataset.py	95.45% <ø> (ø)	⬆️
neuroscout/schemas/dataset.py	100% <ø> (ø)	⬆️
neuroscout/models/task.py	94.11% <100%> (+0.36%)	⬆️
neuroscout/schemas/task.py	100% <100%> (ø)	⬆️

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