Add Audio Block

[timmyc]

For #804

This branch adds the basics of the audio block. Currently you can select/upload an audio file from the Media Library, or use an external url. As mentioned on the ticket, the audio widget supports additional parameters like adding additional playback types, and looping - which we might consider in future iterations.

Also, perhaps there could potentially be some simple validation on the url field - but taking a page from the video block and trying to simplify things for now.

To Test

• Create an audio block using an existing mp3 in your media library
• Create an audio block by uploading a new audio file to your media library
• Create an audio block using an externally hosted mp3 file.

[BE-Webdesign]

Looks good. The extra class works, on the front end but not the back end, don't think that is related to this PR though. One of the tests is not cleaning up correctly and failing. We might need to coordinate with #2309, as the problem lies across both of them. Going to push the fixtures to this branch.

[codecov]

Codecov Report

Merging #2299 into master will increase coverage by 0.02%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master    #2299      +/-   ##
==========================================
+ Coverage   25.82%   25.84%   +0.02%     
==========================================
  Files         156      157       +1     
  Lines        4829     4840      +11     
  Branches      815      817       +2     
==========================================
+ Hits         1247     1251       +4     
- Misses       3024     3029       +5     
- Partials      558      560       +2

Impacted Files	Coverage Δ
blocks/library/audio/index.js	36.36% <36.36%> (ø)

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[mkaz]

Tested it out and it worked well for me on inserting from Media Library, but an issue when trying to type in a URL. It would insert immediately on first letter type and fail since h is invalid URL.

Also, you will probably want to add an Upload button next to the Insert, similar to how Image & Gallery are now doing but that probably can come with a version 1.1

[timmyc]

Indeed, good catch @mkaz - thinking we should emulate the embed functionality for verifying urls: