Use image preview to display the cover image recently uploaded.

[Ruk33]

Fixes

Issue Number: #7159

What is the current behavior?

When the image gets uploaded, it doesn't mean the image
is immediately available to be displayed (that's how the
upload image service seems to work) If we try to display
the image before it's ready, the image will result in
an error that locks the submit button.

What is the new behavior?

When the image gets uploaded, instead of waiting for the
upload image service, we just display a local preview using
base 64. This way, the preview becomes available much faster
and the error no longer occurs.

Other information

PR Checklist

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What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting)
• Refactoring (no functional changes)
• Documentation changes
• Other - Please describe:

Please check all that apply to this PR using "x":

• I have checked that this PR is not a duplicate of an existing PR (open, closed or merged)
• I added a line describing my change to CHANGELOG.md
• I have checked that this PR does not introduce a breaking change
• This PR introduces breaking changes and I have provided a detailed explanation below

[jessopb]

I wasn't able to see any kind of preview on the second or third channel publishes testing this branch. However, I was unable to repro the problem on master.
Did it repro for you?

[Ruk33]

Yes @jessopb, if you upload and then try the URL right away, you may encounter the error described in the ticket. After that, since the image will fail, the error appears.

[jessopb]

Did you test a second channel publish while first was pending?

[jessopb]

@Ruk33 What I did was publish a channel with a cover image. Then I scrolled to the pending channels (at the bottom :( ). I navigated to the pending channel, there was a cover image.
Then I published a second channel with a different cover image.
The second channel's cover image was blank while pending.