Move vol opps search logic from the client side to the server side

Presently, constructing a search on the client side is onerous and fraught with problems. At least the following issues would be resolved by moving this business logic to a single server-side API:

• permissions management would be improved; instead of making permissions exceptions for core API calls, we could manage access to a single volunteer-provided API
• stability: changing a single API call in the current workflow from get to getsingle, for instance, would necessite a change in how permissions are handled
• promise management - if we move this stuff to the server side, we'll probably make just one API call and get everything we need rather than have to chain together several calls
• a custom API would give us the ability to OR parameters; e.g., start_time BETWEEN X and Y, OR end_time BETWEEN X and Y