fixes bug 948704 - date changing links should keep query string intact

@AdrianGaudebert r? (CC @rhelmer)

According to https://bugzilla.mozilla.org/show_bug.cgi?id=948704 there's a bug in how the URL changes when all you try to do is to change the `days=XXX` query string part. Instead of fixing that particular case; I think it's a flawed system that that it can even be a bug. Just changing the `days=XXX` part shouldn't have to be aware of what page it's on and what other query string variables are at play. 

As of this change, it just does that. Change `days=YYY` to `days=XXX` independent of anything else. It basically keeps it simple that way. 

I make the `change_query_string` template function quite advanced with the ability to remove query string keys. This is unrelated to the bug at hand but comes in useful and is actually used on the exploitability page where instead of setting `/reports/exploitability/?page=1` to go back to the "home page" it instead changes it to `/reports/exploitability/`.