Add ability to navigate in the list of events

The list of events displayed in the "Latest Changes" section of the AntiSpam home page is limited to the first 50 events and there is no way to see the next ones from the UI. The only workaround is to manually add an offset parameter to the URL (introduced in ANTISPAM-55), which is not discoverable.

This makes it impractical to review and clean spam events older than the 50 most recent ones.

Add navigation buttons below the event list:

• "More Events", displayed only when there are more events to display
• "Previous Events", displayed only when not on the first page
• Both keep the currently selected filters
• Changing a filter goes back to the first page, since the filtered list is a different one
• Deleting selected events stays on the current page

Also make the offset URL parameter robust: it is a String coming from the request while the event query expects a number, and a non-numeric or negative value is now ignored.