Implement search bar in mainPage

The last discussed mockusps in pagure.io/design/issues/715 featured a large search bar with filters at the top. It would be good to implement this.

While there isn't visual hierarchy in the mockup that separates the search bar from the issues, I wanted to suggest designing the site in such a way that the search bar is always partially visible as the user scrolls through content. For this idea to not devour too much visual real estate, it should shrink up to the header a bit but still be quick for a user to get to.