light refactoring of SourcesController.
extract duplication from SourcesController#create and #update.
initial extraction of Source::Importer.
minor refactor of Source::Importer.
simplify interface between SourcesController and Source::Importer.
move failure message over to Source::Importer.
extract source import success message into a template.
refactor source importing.
Merge pull request #181 from botandrose/refactor_sources_controller

Refactor SourcesController