feat(Response): Add viewable_as property

[tipabu]

Summary of Changes

This is very similar to Response.downloadable_as, but uses an inline disposition type to display in-browser while still offering a suggested filename when saving.

Related Issues

Closes #1951

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[codecov[bot]]

Codecov Report

Merging #1958 (eeab9e7) into master (d1a8bbb) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head eeab9e7 differs from pull request most recent head 828e910. Consider uploading reports for the commit 828e910 to get more accurate results

@@            Coverage Diff            @@
##            master     #1958   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         6616      6618    +2     
  Branches      1067      1067           
=========================================
+ Hits          6616      6618    +2     

Impacted Files	Coverage Δ
falcon/response.py	100.00% <100.00%> (ø)
falcon/response_helpers.py	100.00% <100.00%> (ø)

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