R1:8ec413b97201

Clean up "ids" and "phids" handling in SearchEngines

Summary:
Ref T9964. I added several hacks to get these working. Clean them up and pull this into a proper extension.

The behavior in the web UI is:

- they work in all applications; but
- they only show up in the UI if a value is specified.

So if you visit `/view/?ids=1,2` you get the field, but normally it's not present. We could refine this later. I'm going to add documentation about how to prefill these forms regardless, which should make this…