Hide attributes related to downloading files.

[jkmarx]

Resolves #3277

• Hide ToDo Question. is_exposed is currently used to control the table columns viewed
• Download_url should not be a facet
• Exclude the download and data file in the table config (they are not seen and used only to download files)

[codecov]

Codecov Report

Merging #3382 into develop will decrease coverage by 0.09%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           develop    #3382     +/-   ##
==========================================
- Coverage    68.68%   68.59%   -0.1%     
==========================================
  Files          371      371             
  Lines        24518    24259    -259     
  Branches       942      942             
==========================================
- Hits         16841    16640    -201     
+ Misses        7677     7619     -58

Impacted Files	Coverage Δ
refinery/data_set_manager/models.py	83.08% <0%> (+0.37%)	⬆️
refinery/data_set_manager/views.py	63.42% <100%> (-4.08%)	⬇️

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