Add activity heatmaps

This MR adds heatmaps for Xonotic servers, showing when they are most active. This can be done without filters to show when "the game" is most active overall as well as on a per-server basis. The intensity of each cell in the heatmap is determined by the count of recorded games during that hour of the week, UTC time. As the user hovers over individual cells in the chart, the day and time (in UTC) are shown along the top for reference.

Here's what it looks like:

The data is sourced from the "recent games" materialized view table, which contains summary information about the games played across all servers for the past 30 days.