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

lib/mix/tasks/pleroma/relay.ex

@@ -4,7 +4,7 @@
 
 defmodule Mix.Tasks.Pleroma.Relay do
   use Mix.Task
-  alias Mix.Tasks.Pleroma.Common
+  import Mix.Pleroma
   alias Pleroma.Web.ActivityPub.Relay
 
   @shortdoc "Manages remote relays"
@@ -24,24 +24,24 @@ defmodule Mix.Tasks.Pleroma.Relay do
   Example: ``mix pleroma.relay unfollow https://example.org/relay``
   """
   def run(["follow", target]) do
-    Common.start_pleroma()
+    start_pleroma()
 
     with {:ok, _activity} <- Relay.follow(target) do
       # put this task to sleep to allow the genserver to push out the messages
       :timer.sleep(500)
     else
-      {:error, e} -> Common.shell_error("Error while following #{target}: #{inspect(e)}")
+      {:error, e} -> shell_error("Error while following #{target}: #{inspect(e)}")
     end
   end
 
   def run(["unfollow", target]) do
-    Common.start_pleroma()
+    start_pleroma()
 
     with {:ok, _activity} <- Relay.unfollow(target) do
       # put this task to sleep to allow the genserver to push out the messages
       :timer.sleep(500)
     else
-      {:error, e} -> Common.shell_error("Error while following #{target}: #{inspect(e)}")
+      {:error, e} -> shell_error("Error while following #{target}: #{inspect(e)}")
     end
   end
 end