Counterpart to backend ticket jaegertracing/jaeger#423

The webui does not presently permit a search to be run until a service is selected.

This is awkward; the UI must often be reloaded to see new services. More importantly it makes it impossible to search for a tag or span name across all services. This becomes especially important if you're sharding your services to deal with #38 .

On a related note, the field for the span type that appears once a service is selected doesn't seem to accept freeform entries; if you put something in that doesn't match its validation list, it'll ignore it and find all span types. That's confusing ui and impractical for users who may have lots of span types in a complex system.