Improve instant search partial queries

[LeoMcA]

Currently if a search gets sent midway through a word some pretty terrible (or no) results can be returned.

For instance, "fir" (imagine I was midway through typing firefox) returns no results.

We might want to change how our instant search sends queries automatically. We could:

• Append a wildcard to the end of any query ending in non-whitespace
• Determine if a user is still typing, and only submit queries on whitespace if they are (e.g. a search for "uninstall firefox add-ons" would submit the queries "uninstall" "uninstall firefox" and "uninstall firefox add-ons")
• Something else?

We may want to only enable this for a proportion of users to begin with, especially if we start using wildcards, to ensure we don't put elastic under heavy load.

[akatsoulas]

Closing this in favor of #707 . Both can be tackled and tracked into a single issue

[LeoMcA]

Reopening, we don't want to approach this until we're fully migrated to ES7.