Simplify updating user's `info`

Add User.change_info/2 and User.update_info/2

Before this MR:

def unblock(blocker, %{ap_id: ap_id}) do
  info_cng =
    blocker.info
    |> User.Info.remove_from_block(ap_id)

  cng =
    change(blocker)
    |> put_embed(:info, info_cng)

  update_and_set_cache(cng)
end

After this MR:

def unblock(blocker, %{ap_id: ap_id}) do
  update_info(blocker, &User.Info.remove_from_block(&1, ap_id))
end

Other changes

• Do not return tuple when unneeded. Some functions (such as User.get_friends/1) were always returning a success tuple ({:ok, _}) which complicates the code and confusing.
• Cleanup Pleroma.User