Download images into media folder when importing screenshots

[BernhardPosselt]

When importing screenshots at https://github.com/nextcloud/appstore/blob/master/nextcloudappstore/core/api/v1/release/importer.py#L134 the real screenshots should be downloaded into the media folder at MEDIA_ROOT/APP_ID/screenshots/NUMBER.EXT where

• MEDIA_ROOT is the media root defined in the settings file (default should be media/ in the main root)
• APP_ID is the app's id
• NUMBER is the screenshot position
• EXT is the extension. Use https://docs.python.org/3.5/library/mimetypes.html to parse it and validate it against a configurable setting in base.py, default: ALLOWED_IMAGE_MIMES = ('jpg', 'png', 'gif')

Example: media/news/screenshots/1.jpg

Previously stored images should be removed upon successful import

In addition the maximum screenshot size should be limited to a configurable setting in base.py, default: MAX_DOWNLOAD_IMG_SIZE = (1024*2) 2 # 2 MB To achieve that refactor https://github.com/nextcloud/appstore/blob/master/nextcloudappstore/core/api/v1/release/downloader.py#L28 so that a downloader can download any file

The image URLs should then be changed to relative path from the MEDIA_ROOT, e.g. news/screenshots/1.jpg. Finally serializers and the web interface need to be adjusted

Deleting apps ofc also gets rid of the screenshots

[nursoda]

Not sure why this is still open as images seem to be cached meanwhile (see #925). Current solution seems to be a redirection (dynamic loading through the app store).