Add faceted search for license and source

[adamreichold]

The source field is not really amenable for faceted search directly, so I introduced an additional "provenance" field which can be used to hierarchically model where our data is coming from.

Closes #61

[adamreichold]

The string interning is probably premature optimization, but we have string_cache in our dependency closure anyway (via scraper) and it integrates nicely with serde, so why not use it.

[adamreichold]

So this provides a simple faceted search for the (open) licenses, i.e. results include license count:

Clicking one of the license facets will append the corresponding term to the query and resubmit the form to update the search results:

[adamreichold]

Just another pair of screenshots of how this works now.

The initial search UI

and after clicking first "/open" and then "/open/dl-de"