Followup to r83715: use internal HTTP client, handle exception when previewing
- Replace own HTTP client with the generic HTTP::get (as per code review)
- SpecialBannerLoaderException needs to be handled when previewing banners. This exception needs to handled in the user interface, currently a newly introduced 'centralnotice-nopreview' message is displayed in case of banner generation failure.
- Unfortunately, UI code is full of duplication and should probably be refactored.
- getDonationAmount() and friends should factored out of the SpecialBannerLoader