Add document liking

[rtjoa]

Users should be able to like others documents so we have a way to sort the explore page by quality.

It would also be cool to be able to sort by "Top" (most likes) and "Hot" (formula like this).

Some other considerations about document discovery:

• Perhaps we also have different types of likes, such as useful/interesting/funny
• With just basic liking, we could implement recommendations, e.g. "Documents you might be interested in"
• Document tagging, by the creators (e.g. most articles) and/or the consumers (e.g. Steam games)