better full text search: build with paragraph or page

[STRRL]

currently contents in table text_fts are stored at the "word" level. so when we search content with multi-word keywords, there would be mostly no results.

I propose a new way to build the full-text search feature:

• build the table text_fts as the "page" level, it means all the content in one screen shot would be stored in one row.
• use the same query to search the result, find the images
• for each image in the previous step, find the available keyword, and markup it.