Add missing variables to search page

[simonwo]

• is_tag_search
• browse_by_tags_link

And l:pages for search results.

Search results should contain:

• newchapter
• chapterend
• chaptername
• chapterdescription
• and a bunch of fields around thumbnails

There is some clear duplication here with other lists of things, so check out whether another common drop is appropriate.

[simonwo]

The second part is done in ebcbc1c, we still need the first part and [l:pages], if it actually exists.

[simonwo]

[l:pages] is done in 9893b84.