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[SteamGifts] Additional giveaway search tools

Fixes header to the top of the page so it "moves with it" and adds a search input to it. In addition, adds a "more like this" button close to giveaway's image for searching by item (same as small icon close to the game's name, but more usable).

当前为 2017-01-12 提交的版本，查看 最新版本。