Revisit codebase: Strip unnecessary features, general cleanup.

[Treora]

• Stop separating the concepts of visits and pages; we now just store pages, and drop the ambition of capturing the full browsing history (at least for the time being).
• Accordingly remove unused code for deduplication of pages; no need for that if the user only stores pages manually, and it complicated things a lot.
• Stop showing adjacent snapshots to provide context in search results; it was rudimentary and not worth its complexity.
• Remove the bit of pdf search support we had.
• Remove support for searching in omnibar. Seems unnecessary, mediocre UX, slightly buggy.