Add a link proxy to URLs outside of Retrospring

[pixeldesu]

Internally, we reconsidered adding the ability to actually parse links in questions, no matter the origin.

But something we definitely need to add for this to work is adding a "Link Proxy", which basically shows a warning page akin to "You're leaving Retrospring for [full url], click here if you are sure".

Example: https://steamcommunity.com/linkfilter/?url=https://retrospring.net

It just can be a simple action in StaticController taking the target URL as an argument.

Something up for discussion should be the URL/route for the action? If you have an idea, feel free to propose something.