Allow to upload more than one study consent

There may be cases in which a study uses more then one consent document. Cases may include:

• The study goes for a long time and they want to refresh the consent for the upcoming phase. Uploading the document for each phase individually is much simpler then downloading the old one, appending the new pages, and uploading the combined file.
• The study has multiple, optional parts. Uploading the files individually allows users to quickly see which parts the subject has agreed to.

I considered two general approaches:

• Structured: Study managers must define which kinds of consents (and how many) are expected. We can then use that information to interpret the uploaded documents, e.g. show something like "3/5 consents uploaded" or make judgements whether we are allowed to process this subjects data.
• Unstructured: Just allow to upload more then one file, but do not make any judgements.

The study consent is currently not used in Castellum other then being displayed. So the unstructured approach is the better fit right now. However, if we ever want to make judgements and therefore need more structured data, migrating that way is hard. I expect that most studies will use a single study consent. But the studies that actually use this feature would have to be migrated manually.