zendesk: store locally the Zendesk user id

We can't rely unfortunately on the search endpoints, due to an indexing
lag. Instead, we keep the kb_account_id -> zd_user_id mapping in a
table locally.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
db: add unique constraints on kb_account_id and zd_user_id

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>