refactor(backend): change naming of function names to avoid the is_ prefix

Following Credo.Check.Readability.PredicateFunctionNames check

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/graphql/resolvers/participant.ex

@@ -384,7 +384,7 @@ defmodule Mobilizon.GraphQL.Resolvers.Participant do
     with {:member, true} <-
            {:member,
             to_string(current_actor_id) == to_string(actor_id) or
-              Actors.is_member?(current_actor_id, actor_id)},
+              Actors.member?(current_actor_id, actor_id)},
          {:ok, _activity, %Conversation{} = conversation} <- Comments.create_conversation(args) do
       {:ok, conversation_to_view(conversation, Actors.get_actor(actor_id))}
     else