Remove hard coded channel IDs

Migrated from https://github.com/dotnet/core-eng/issues/10120

@ChadNedzlek wrote:

https://github.com/dotnet/arcade-services/blob/ae5fa543ba2369b12581c1905970910ff2a1f927/src/Maestro/Maestro.Web/Api/v2018_07_16/Controllers/ChannelsController.cs#L250

These values are not the same between int and production, but shouldn't be hard coded anyway.

This code makes this API entirely untestable in int. We should at least just fetch those and cache them. But given that these calls make hundreds of DB requests in the end, we should just add one more and look up the right answer each time.