Save series_id for submissions

[jorg-vr]

This pull request saves the series_id when creating a submission or an activity read state.

It also limits visibility of hidden series to course_members.

This series ID is used for:

• better links to the activity in series in course from a submission (defaults to previous behavior if no series present)
• determining the next suggested activities on the homepage (will only suggest course if no series_id present)
• validating if activity is accessible within that specific series when submitting (defaults to previous behavior if no series present)

This ID currently isn't used for the activity status. This is planned for a next step, but we'll also need a decent change for existing submissions.

Next steps:

• Use series id for activity status
• Handle removal of series/removal of activities from series
• Update existing submissions or correctly handle submissions without series_id

• Tests were added

Part of #5548

[pdawyndt]

While working on this issue, we might also consider storing some other submission properties that are now missed. It concerns those properties that are passed to the judge upon submission. One such property is the language setting of the user upon submission. As this is not stored, we no longer have that information upon re-evaluation. As a result, the language setting of the person re-evaluating is taken as the language passed to the judge upon re-evaluation (which changes the feedback in that respect).

[niknetniko]

Besides the single comment, the code looks good.

Do we also want to include this information in the JSON API, or is it better to do this later?