- fix vulnerability in rewrite_relative_url() which prevented some URLs from being properly absolutized
- fetch_file_contents: validate all URLs before requesting them
- validate URLs: explicitly whitelist http and https scheme, forbid everything else
- DiskCache/cached_url: only serve whitelisted content types (images, video)
- simplify filename/URL handling code, remove and consolidate some less-used functions
* DiskCache: multiple fixes; support isWritable() for cache entries, set content-disposition for send()
* public/cached_url: allow selecting files from sub-caches other than images
* plugins/Cache_Starred_Images: rework to use DiskCache, can be enabled per-user, properly handles article enclosures, etc