Implement fetching announcements from zulip

### Is your feature request related to a problem? Please describe.
As a user I want to have integrated zulip announcement so I could see most recent on homepage

### Describe the solution you'd like
Implement Zulip API to fetch and display announcements.

### Describe alternatives you've considered
Add additional source to zulip.

### Additional context
Key to Zulip must be stored securely. It might be that there will be a need to implement hooks and cron deployments.