Automatically enters giveaways from Steamgifts.com and site enhancements

Extension adds some features to SteamGifts.com:

• AutoJoin (join everything you see on the page)
• Infinite scroll (giveaways load dynamically as you scroll)
• Button to Join or Leave giveaway on the main page
• Improved hiding giveaway: no need to confirm and reload page
• Hide all DLC giveaways
• Night Theme

And some other features

Github page: https://github.com/ge-ku/AutoJoin-for-SteamGifts"