Add reactions and user notes

A feature proposal from the BeLEARN project

- Implement a reactions system (e.g. slack, telegram) which can be saved and shown on each item
- Note-taking system: each user can store and read its own user notes for each item
- Future iterations could implement a more complex highlighting system, which could allow adding notes to specifically selected content (text, images). This would however require complex tracking logic when items are e.g. edited (what happens if a selected text is truncated?)