Add API pass-through endpoint for diffs

[Mr0grog]

We probably want to eventually have a more nuanced way to calculate diffs (see also: “scraping notes” on edgi-govdata-archiving/versionista-outputter#4 (comment)). This should probably be a separate service and should be configurable pluggable.

See also https://github.com/edgi-govdata-archiving/differ

[Mr0grog]

From @danielballan on #42:

web-monitoring-db will broker responses from "diffing services"

• diff/start..end/ -> UNIX-style diff
• diff/start..end/pf -> PageFreezer JSON response
• maybe temporarily support diff/start..end/versionista but only versionista-source pages would support that

Response formatted as JSON. Maybe support richer types later.

{'page_id': ...,
 'version_id', ...,
 'diff_service': ...,
 'diff_service_version': ...,
 'content': VERBATIM_RESPONSE_FROM_DIFFING_SERVICE}