Add search_type parameter to Web and API

Problem to solve

In order to continue the Zoekt production rollout safely we need to split web UI and API for Zoekt.

After this change we're going to have web using Zoekt (if available) and API using advanced search. That way we won't break customer's automations and/or cause incidents.

Proposal

As part of this issue we need to add search_type parameter to Web and API. It can be set to 3 options:

• basic (we could skip it potentially)
• advanced
• zoekt

The defaults should be set to zoekt for web UI and advanced for API.

Q: Do we want to set zoekt as default for SM customers?