This diff uploads the local thumbnail generated on the `native` client to the `keyserver` where the contents will be persisted in the `uploads` table of the `serverDB`.
NOTE: This diff can be landed as-is. The entry's `container` column will be `NULL` since there's plenty of work we need to do in `multimediaMessageCreationResponder(...)` before we're ready to call `assignMedia(...)` or the equivalent to set the `container` value. However, it's still fine to add these entries to the `uploads` table because they'll be swept up by the `deleteUnassignedUploads` (keyserver/src/deleters) cron job that runs every day at 3:30AM PT (keyserver/src/cron/cron.js`).