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/database.ex

@@ -3,12 +3,12 @@
 # SPDX-License-Identifier: AGPL-3.0-only
 
 defmodule Mix.Tasks.Pleroma.Database do
-  alias Mix.Tasks.Pleroma.Common
   alias Pleroma.Conversation
   alias Pleroma.Object
   alias Pleroma.Repo
   alias Pleroma.User
   require Logger
+  import Mix.Pleroma
   use Mix.Task
 
   @shortdoc "A collection of database related tasks"
@@ -45,7 +45,7 @@ defmodule Mix.Tasks.Pleroma.Database do
         ]
       )
 
-    Common.start_pleroma()
+    start_pleroma()
     Logger.info("Removing embedded objects")
 
     Repo.query!(
@@ -66,12 +66,12 @@ defmodule Mix.Tasks.Pleroma.Database do
   end
 
   def run(["bump_all_conversations"]) do
-    Common.start_pleroma()
+    start_pleroma()
     Conversation.bump_for_all_activities()
   end
 
   def run(["update_users_following_followers_counts"]) do
-    Common.start_pleroma()
+    start_pleroma()
 
     users = Repo.all(User)
     Enum.each(users, &User.remove_duplicated_following/1)
@@ -89,7 +89,7 @@ defmodule Mix.Tasks.Pleroma.Database do
         ]
       )
 
-    Common.start_pleroma()
+    start_pleroma()
 
     deadline = Pleroma.Config.get([:instance, :remote_post_retention_days])