Add thumbnails to card items

[SeanNaren]

What does this PR do?

Adds thumbnails to the card items. @ethanwharris to help me out get them onto S3 :)

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• Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #787 (262f310) into master (d190c76) will decrease coverage by 3.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
- Coverage   89.62%   85.76%   -3.87%     
==========================================
  Files         213      213              
  Lines       11317    11317              
==========================================
- Hits        10143     9706     -437     
- Misses       1174     1611     +437     

Flag	Coverage Δ
unittests	85.76% <ø> (-3.87%)	⬇️

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

Impacted Files	Coverage Δ
flash/text/question_answering/model.py	29.16% <0.00%> (-61.81%)	⬇️
flash/text/question_answering/data.py	28.29% <0.00%> (-58.53%)	⬇️
flash/text/classification/cli.py	53.84% <0.00%> (-46.16%)	⬇️
flash/text/question_answering/cli.py	60.00% <0.00%> (-40.00%)	⬇️
flash/text/seq2seq/core/utils.py	70.00% <0.00%> (-30.00%)	⬇️
flash/text/classification/data.py	62.92% <0.00%> (-28.09%)	⬇️
flash/text/seq2seq/core/metrics.py	74.07% <0.00%> (-25.00%)	⬇️
flash/text/seq2seq/core/data.py	65.85% <0.00%> (-24.40%)	⬇️
flash/text/classification/model.py	69.09% <0.00%> (-23.64%)	⬇️
flash/text/question_answering/finetuning.py	5.49% <0.00%> (-14.29%)	⬇️
... and 3 more

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[tchaton]

LGTM !

[ethanwharris]

LGTM 😃