Add Public/Private Document Split

[ericpan64]

Give users the option to have publicly listed documents they're learning from on their profile. Default to private

Ideally there are 3 levels of publicity:

1. Entirely public (anyone can see it)
2. Partially public (e.g. sharable via link, though unseen otherwise)
3. Private (only authenticated self can see it)

[ericpan64]

Once this is implemented, there are 2 good follow-up issues to consider:

• Searchable library for public docs: think through how to store this and query using Elasticsearch
• User Security for public docs: need to make sure people aren't able to do malicious stuff via shared docs. One option to consider is a base URL whitelist that's at the user-level