Submission metadata feature requests

Hi! I have two proposed feature requests for submission metadata:
1. The "View Metadata" bubble on the leaderboard currently only displays the `submission_meta_attributes` from the config and not `default_submission_meta_attributes`. I would prefer `default_submission_meta_attributes` (e.g. project URL and publication URL) to be visible as well, or at least for there to be an option to make them visible.
2. When I make a submission and decline to provide information for `submission_meta_attributes`, there is currently no place to add/edit those fields after submission. The Edit option on the My Submissions page only displays the `default_submission_meta_attributes` if no `submission_meta_attributes` were included with the original submission (images below). I'd like it to be possible to fill those fields in after submission.

Edit submission pop-up after submitting *without* `submission_meta_attributes` for challenge 1256:

Edit submission pop-up after submitting *with* `submission_meta_attributes` for challenge 1256:

Start by tracing the submission metadata handling behind the leaderboard's View Metadata bubble and the Edit option on My Submissions, using the challenge 1256 examples in the issue as the expected cases. Done means default metadata can be shown in the leaderboard view and omitted submission metadata fields can be added or edited after submission.

Written by the indexing model from the issue text.