Create channels URL from window.location

[medihack]

Closes #20068

What I did

Create channels URL dynamically on the client by using the window location object.

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
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  MIGRATION.MD

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["bug"]

[medihack]

I simply got rid of SERVER_CHANNEL_URL. I don't see a case where it is really needed anymore, but not sure about that.

[medihack]

Hm, something is strange with the commit history here. Creating a new PR.