Lower memory usage in submissions and projects

[frjo]

Fixes #4520

This are not dramatic changes but many small things adds up. In testing the savings are small but I think that on large live environments they can be noticeable.

• Defer loading to have fields like search_data, search_document, form_data and form_fields only load when needed.
• Use submission_id instead of submission.id to avoid a db query when the model is not loaded/needed.
• Implement select_related() to avoid N+1 queries.
• Cache get_object() so it is not loaded multiple times in one request.

[frjo]

@wes-otf I have put this on test now so we can make sure it doesn't break anything.