Cache job indicator by key.

Fixes #534.
* Sets indicator job ID in raster cache key
* Refreshes WMS tile layer when selected city changed
Delete jobs raster cache with indicator job.

Trigger celery task after IndicatorJob is deleted to remove its related cached jobs indicator raster files.
Merge pull request #561 from flibbertigibbet/feature/job-indicator-cache-key

Cache job indicator by key.