This is needed for multi-file uploads. We need to know both the image
url and the page url to set the post's source correctly when converting
an upload media asset into a post.
Make upload_media_assets.media_asset_id nullable in order to support
multi-file uploads. The media asset will be null while the image is
still being downloaded from the source.
* uploads.media_asset_count - the number of media assets attached to this upload.
* upload_media_assets.status - the status of each media asset attached to this upload (processing, active, failed)
* upload_media_assets.source_url - the source of each media asset attached to this upload
* upload_media_assets.error - the error message if uploading the media asset failed
Add a join table that allows multiple media assets (images or videos) to
be attached to uploads. This is for a future ability to upload multiple
files at once.
