Add public data files that belong to projects

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Description

This comes out of a discussion with people running projects and @madprime and I discussed it this morning (actually their idea, not mine. I just wanted to have it as an issue).

People running projects on Open Humans sometimes want to provide some additional data files to all of their participants and other interested parties through Open Humans. These files are not associated with individual people, but rather are additional files that can be used to analyze the data sets that people contribute to the project.

The solution we settled with: Projects leads that run a project on Open Humans should be able to upload additional data files that will then be associated with their project. These files will be publicly available from the project homepage.

Todo:

Add file-model that belongs to a given project
views for upload/delete these files for the project lead
add the files/download links to the project-detail-view

Open Questions:

do we want to integrate this into the API too?

Related Issue(s)

Example

E.g. for my Twarxiv project I might want to provide a list of the "most common hashtags around the globe" or "background frequency of emoji use world-wide" so that people can use these data sets to further dive into their own Twitter archives. If I could upload this list to my Open Humans project I could easily link to these files and they would be visible to everyone checking out the project on our website.

OpenHumans / open-humans