Feature: Lists (for bookmarking posts and comments)

[previnder]

Implements Lists as discussed in #21.

Tasks:

• [x] Backend:
  • [x] Migrations.
  • [x] The core functions.
  • [x] API endpoints.
• [x] UI:
  • [x] Add-to-list modal.
  • [x] Lists page (at the location /@user/lists).
  • [x] List page (at the location /@user/list/favorites).
  • [x] Lists in the sidebar.
  • [x] Connect to the API and store fetched data in Redux.

Closes #21.