Add Pleroma.Utils.command_available?/1 and use where appropriate

lib/pleroma/utils.ex

@@ -9,4 +9,19 @@ defmodule Pleroma.Utils do
     |> Enum.map(&Path.join(dir, &1))
     |> Kernel.ParallelCompiler.compile()
   end
+
+  @doc """
+  POSIX-compliant check if command is available in the system
+
+  ## Examples
+      iex> command_available?("git")
+      true
+      iex> command_available?("wrongcmd")
+      false
+
+  """
+  @spec command_available?(String.t()) :: boolean()
+  def command_available?(command) do
+    match?({_output, 0}, System.cmd("sh", ["-c", "command -v #{command}"]))
+  end
 end