Pipelines create: fetch list of user repos to push the pipeline

[mirpedrol]

          Suggestion: I wonder if many people will create an empty GitHub repo _before_ running `nf-core pipelines create` (this is probably what I would do). Then we have to rely on people reading the help text printed by the command, which is difficult.

We could add functionality to fetch a list of the user's GH repos (maybe filtering for repos that have zero commits?) and then offer to push to one of them via a dropdown.. Might not be that difficult to do, and could help streamline that final step a little more.

Originally posted by @ewels in #2404 (comment)

[mirpedrol]

Closing this as it is taken into account already: #2404 (comment)