Polylang fixes, and additional stuff

• Attempt to fix issue with polylang tags being inserted in the wrong langauge.
• Add random no-cache parameter to many admin requests (could help bypass certain types of caching)
• Option to drop custom tag slug prefix (mfn-cus-)
• Option to rewrite mfn-news-tag URL slug, similar to what we do with post type.
• Fix delete all tags for polylang
• A bit more localized tags
• Add classes "mfn-primary" and "mfn-image-primary" for attachments in the Attachments Widget
• Other fixes