Change tag search input to be more user friendly

[yurishkuro]

Currently tag query must be entered into a single textbox with tag:value syntax, which is not obvious and too sensitive to white space. Also doesn't seem to work for multi-tag search at all.

The suggestion is to replace a single box with "add tag query" button which adds two text boxes side by side, for tag name (can be used with auto-complete in the future) and tag value. The button allows adding more tag queries. Each pair of key-value text boxes should have a red X to remove it from the search.

[vprithvi]

Related: https://github.com/uber/jaeger/issues/139 (indexing values for autocomplete)

[tiffon]

@yurishkuro Would you say the intent of this ticket has been addressed now that the tag input on the search form handles logfmt values?

[yurishkuro]

Fixed by #147