Rename Pleroma.Mix.Tasks.Common -> Mix.Pleroma and import it's functions

Rename Pleroma.Mix.Tasks.Common -> Mix.Pleroma and import it's functions
instead of aliasing

This seems to be the convention for functions that can be reused between
different mix tasks in all Elixir projects I've seen and it gets rid on
an error message when someone runs mix pleroma.common

Also in this commit by accident:
- Move benchmark task under a proper namespace
- Insert a space after the prompt