Show time of last crawling operation in /feed and /events

[sic2]

• Show time of last crawling operation in /feed and /events
• Display the information of how often the crawler runs
• Have a page refresh button that activates if the page may contain new data

[vigliag]

We now have a last_crawled_at column in the Source event. This information could easily be taken from there. Selecting the highest value of that column should be enough. I'd skip adding the refresh button however