Create API call to the activity of a specific user

[yoavartzi]

We need an API call to get the activities of a specific user, so we can display them on the user profile page. Activities include recommendation and reactions. Activities will be displayed in a reverse chronological order (most recent at the top). Activities will be displayed on the user page, interleaving recommendation and papers in the order they appear. The API should allow the same continuous scrolling currently available on the user profile page.

The activities returned will respect the recommendation cycle, so a recommendation is not released to be returned by this API until the cutoff time (and then its time is the cutoff time).