Fixes #964

Testing done

Tested on a large workspace (~1900 channels) where this only worked for 1 file upload before it got rate limited before.
It now works as long as you don't clear the cache and then immediately try upload a file.

I added in some functionality to refresh when a channel isn't found so that new channels are picked up immediately, to de- risk it I made it not invalidate the old cache on errors.

I've tested it quite thoroughly, but its quite painful, really annoying of slack to implement it like this...

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