Add support for raster filed on import

[jonpmarino]

As a user, I want to upload raster data, such as a print map or an aerial image, or sequence of images, into MapStory as a StoryLayer so I can do things, like, make MapStories that animate digital data on top of historic print maps or images.

Acceptance Criteria

• Support injest of a single geo-referenced image with a date
• Support injest of a zipped file with several images (mosaic) showing change over time

Notes
Previous tickets for raster upload on the old site:
MapStory/mapstory-meta#669
MapStory/mapstory-meta#1264

[jonpmarino]

Now that os-geo importer is up to latest, we can investigate the state of raster support that is supposed to be there

[Coop56]

django-osgeo-importer does support georeferenced raster files. We need to make some business decisions around this about supporting raster without time. Closing this specific ticket as it's complete.