Expire caches in merge API

Merge API did not expire caches causing changes not to be shown.
Return JSON payload also from merge API

Both import and update API endpoints return {'ok': 1} if all went well.
For consistency return such message also from the merge endpoint.
Catch timeout errors from external services

Network requests to external services (Bugzilla) may time out. Catch
those and handle as not found instead of rendering an error page.