Fixed icon encodings not getting saved

[jayoshih]

Description

Persists save on channel thumbnail to end of the command

It turns out that Django's save function acts more like a POST request in that all fields get submitted, rather than those that have changed like a PATCH request. This was causing an issue as the publish_channel function had one version of the channel and the create_content_database had another version. The create_content_database method would save the icon_encoding. However, any saving after the create_content_database function would then use the outdated model from publish_channel, which caused icon_encoding to get set back to null

This just passes the model itself to create_content_database from publish_channel so they are working from the same object in memory

Issue Addressed (if applicable)

Addresses #1299

Steps to Test

• Remove the thumbnail encoding from a channel
• Publish and check the public channels endpoint

Checklist

• Is the code clean and well-commented?
• Has the CHANGELOG label been added to this pull request? Items with this label will be added to the CHANGELOG at a later time
• Are there tests for this change?

[codecov]

Codecov Report

Merging #1576 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1576      +/-   ##
==========================================
- Coverage   79.49%   79.45%   -0.04%     
==========================================
  Files         252      252              
  Lines       11757    11766       +9     
==========================================
+ Hits         9346     9349       +3     
- Misses       2411     2417       +6

Impacted Files	Coverage Δ
contentcuration/contentcuration/utils/publish.py	77.97% <100%> (-0.21%)	⬇️
...ration/contentcuration/tests/test_exportchannel.py	90.84% <100%> (-2.95%)	⬇️

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