Allow Homarus to generate PNG audio thumbnails

[kayakr]

GitHub Issue: Islandora/documentation#1298

What does this Pull Request do?

Allow Homarus to generate PNG audio thumbnails by adding image/png as allowable request content type and image/png_image2pipe as output target.

What's new?

Homarus can generate jpeg thumbnails. This allows PNG thumbnails, as specified for thumbnailing of audio files.

How should this be tested?

• Apply this config to Homarus: /var/www/html/Crayfish/Homarus/cfg/config.yaml
• Add 'Audio - Generate a thumbnail from an original file' action from Use-a-Thon: Waveform thumbnails for audio files documentation#1298
• Create a repository item with model=Audio, and Audio media with Use=Original file and upload a .mp3 or .wav file.
• In Content administration, check the checkbox for the audio item, then select the action 'Audio - Generate a thumbnail from an original file' and click 'Apply to selected items' button.
• Check the Media tab for the audio repository item, you should see a new media with Use=Thumbnail Image with a related PNG file representing the waveform (black waveform on white background).

Interested parties

@Islandora-CLAW/committers

[codecov]

Codecov Report

Merging #78 into dev will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                dev      #78   +/-   ##
=========================================
  Coverage     94.64%   94.64%           
  Complexity      160      160           
=========================================
  Files             9        9           
  Lines           654      654           
=========================================
  Hits            619      619           
  Misses           35       35

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[dannylamb]

Looks good. Thanks!