Fix #341 - Make channel file in block_actions payload optional (+ add view field)

[seratch]

Summary

This pull request fixes issue #341 that affects TypeScript users. In connection with this, I've added the missing view field for the cases of modals and Home tab.

Also, I noticed block_actions payload in TypeScript doesn't support multi-select block elements yet. I filed another issue #342 and will work on it in a separate pull request.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #343 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #343   +/-   ##
=======================================
  Coverage   73.35%   73.35%           
=======================================
  Files           7        7           
  Lines         503      503           
  Branches      146      146           
=======================================
  Hits          369      369           
  Misses        102      102           
  Partials       32       32

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[stevengill]

Nice fix @seratch!