Resolve "Refactor `Gargantext.Database.Prelude`"
Closes #421 (closed)
This is mostly a rename of the types in Gargantext.Database.Prelude to create some sense of consistency. The convention is documented at the beginning of the module.
I made it so that DBCmd, the command type that was used most in the codebase --- and by far --- kept the same name.
Other than that, I removed the CmdR and CmdRandom types, which felt quite ad hoc and were not used that much. I also tidied up part of the Gargantext.Database.Prelude module, and removed some commented code I found in the codebase.