Support logfmt for tags text input in the search form

[tiffon]

Ref jaegertracing/jaeger#550.

Add a tags query string parameter that is URL encoded JSON.

Tags text input to accept logfmt. Front-end parses this, converts all values to strings, serializes as JSON, and passes over the wire as the value for tags form value (via GET, as-is currently used).

Add a tooltip next to the tags text field that mentions logfmt, etc.