Cache image size (take 2)

Per previous pull request https://github.com/gothfox/Tiny-Tiny-RSS/pull/383

> > Something like _MIN_CACHE_IMAGE_SIZE, which by default would be 1024 but could then be changed per-install for those who want to cache everything.

> Yeah this would be fine. This way paranoid people would tweak "broken SSL" to their hearts content instead of consulting a medical professional and everyone else wouldn't worry about it.

"Broken SSL" is poor wording on my part; "mixed SSL and non-SSL content" is more accurate.

This request changes the cache_images() function to use a constant which can be overridden in the config.php file.