Add redirect to search page on 404 status

[janette]

Fixes #2484

Changes 404 behavior to produce a working search initially set to the bad URL. Also changes the header search box behavior to search whole site, not just datasets.

Steps to reproduce

• Type in a url that does not exist, when on the 404 page, try to use the search block in the header
• Confirm the search is not performed
• Now go to an existing page, use the search block in the header
• Confirm that the results are filtered to only include datasets

QA Steps

• Type in a url that does not exist
• Confirm that you are redirected to the search page
• Confirm that the search block in the header works and does not limit results to datasets