Redesign the feed to switch to a continuous feed and switch to all activities

[yoavartzi]

Currently, the feed only shows one cycle at a time, and the user can switch between cycles using the widget on the left sidebar. We need to-design the feed to show a chronological sequence of activities, including both recommendations and reactions. The new feed should use new API (#386).

1. The feed should be continuous, so cycles should appear one after the other
2. We should have a separator header for each cycle, it can look similar to what we have now to denote the date:

3. Similar to how continuous feeds are built, the feed should load gradually as a user scrolls down (this kind of loading is already implemented in the [all-users](https://recnet.io/all-users) page) 4. We should keep the widget on the left sidebar to jump between cycles, it should just jump to the appropriate cycle