when tile is selected, perform call to /tile endpoint to get tile status

[wscullen]

such as raw_product downloaded or l2a product exists, tile product status will be used to determine if a new job is added when the "Start All" button is pressed

[wscullen]

This relates to #12

s2d2_api should be enhanced to manage tiles and derivative products. Tiles can be queried for their status and which products exist. This would allow smarter submission of jobs and allow multiple people to work on the same area without repeating downloads or processing. Downloads are less of an issue because they are unique, processed products are not unique in that they have the property of the date when they were generated so it is possible to have many duplicates of the same higher level product unintentionally.