Eliminate logging of problematic XML files

As per #416, we're importing XML files not from the input source, but
from when they're output to /htdocs/downloads/code-xml/. So there's no
need to track problematic XML.
Permit per-field filtering

Towards #225, as per the standards established by other API endpoints.
Don't test that cURL is installed

We already do this in ParserController::test_environment(). Per #416.
Don't validate each XML file before indexing it

The XML was just output by the web server a few minutes prior; there's
no need to waste the time validating that each one can be read. Per
#416.
Simplify our index_laws() blacklist import

The process of applying the blacklist of invalid XML files to the Solr
import was too slow. Simplify. Per #416.
Remove path from index_laws' XSLT

Solr doesn't need a complete path for an XSLT—there's only one valid
place where it can exist. Per #416.
Fix a file iteration bug in index_laws()

We were iterating over every file to be imported, rather than every
file in the current slice of that list. Per #416.
Tell cURL that we're posting index_laws() data

Per #416.
Improve how we deal with cURL errors in index_laws()

Per #416.
Tell Solr to commit the indexed laws

Towards #416.
Report to the logger than Solr ran successfully

Per #416.