Check for named binding before joining

[Flo0807]

closes #530

This PR checks for a named binding before joining. It prevents an error being thrown if a join with the same alias has already been performed, e.g. in the item_query function.

However, if we do not explicitly set an alias when joining in item_query this will not prevent a second join from being performed.

Consider a post belonging to a user. If we define an item query like this.

@impl Backpex.LiveResource
def item_query(query, live_action, assigns) do
  query
  |> join(:left, [p], u in Demo.User, on: u.id == p.user_id)
end

maybe_join will do a second join because there is no alias user → has_named_binding/2 returns false.

If we want to prevent a second join to be performed, we have to set the user alias explicitly:

@impl Backpex.LiveResource
def item_query(query, live_action, assigns) do
  query
  |> join(:left, [p], u in Demo.User, on: u.id == p.user_id, as: :user)
end