PlexMediaServer 0.9.9.12.504

Closes #5100
Add Metalab's Flow Cask
Rename metalab-flow cask to only flow

Considering that the the original `flow` cask was discontinued, it
makes more sense to add the vendor prefix since its usage will only
decline
Merge pull request #5098 from gabrielizaias/metalab-flow

Add Metalab's Flow Cask