Core: make Tracker actually use if-modified-since

[black-sliver]

What task needs to be completed?

When loading the tracker twice or doing the XHR update, Firefox will not send the if-modified-since header (when using waitress; have not tried the prod deployment). This should be fixed.

I think the tracker page is missing a cache control header to actually make use of if-modified-since.

Also, while at it, the tracker caching should probably be changed to reply "Not Modified" instead of the cached HTML if it is cached and not modified. Currently the cache overrules the if-modified because using the cache does not require loading the room.