WebSearch: add Open Graph markup
- Enhanced bfe_meta to support knowledge bases and variables.
- Added bfe_meta_opengraph_image.py and bfe_meta_opengraph_video.py to deal specifically with insertion of images and videos.
- Added CFG_WEBSEARCH_ENABLE_GOOGLESCHOLAR and CFG_WEBSEARCH_ENABLE_OPENGRAPH to configure use of Google Scholar or Open Graph protocol markup in the page header.
- Remove duplicate values for a given tag.
- Added demo 'COLLECTION_TO_OPENGRAPHTYPE' knowledge base to map collection identifier to Open Graph object type.
- Other small fixed and enhancements.