Make legacy browsing checking less aggressive

We recently upgraded Suchi, this caused us to target a greater number of older browser. Some of which GOV.UK has substantial usage by. This was most obvious in Chrome 34, which is moving out of supported, but is heavily used (based on new traffic to /help/browsers - linked to from the upgrade bar).

Stats have also shown there are a lot of users who can't upgrade, usually due to strict IT policies. I've increased the dismissal time between seeing the upgrade bar form 2 weeks to 4 weeks, so at least we nag them less often.

https://www.pivotaltracker.com/story/show/73204998