S3 support and better video convertion

[satyamtg]

This changes the following

• Add S3 support for images and videos - The S3 key is as follows to accommodate all types of assets - {fpath.suffix[1:]}/{safe_url}/{quality}, where quality is "high", "low", or "default", and safe_url is a S3 safe version of the URL without the scheme.
• Support Mp4 and WebM both - The --format argument is added which has support for both mp4 and WebM. The --low-quality option is also added. (--autoplay is added in options but not connected with the templates yet)
• Refactored download function to manage S3 in single place - The download and optimization is handled by a single function called download_file() that automatically checks for videos and downloads them from youtube if needed. It also provides us a way to have S3 implemented at one place instead of at different places for images and videos.

The following new functions are added -

• download_file() - Downloads a file from given URL to given path.
• generate_s3_key() - Generate a S3 key given the filename and its URL
• optimize_file() - Send the downloaded file to the correct function to optimize
• convert_video() - Converts a video if low quality required or video not in required format. Uses zimscraperlib.video
• optimize_image() - Optimizes an image based on its type
• download_from_youtube() - Uses youtube_dl to download a file from youtube
• download_from_url() - Download a file using save_large_file(), and return path to the downloaded file. It downloads to a temporary file if the mimetype (from the head request) does not match the required file extension. The temp file has an extension same as the mimetype.
• get_meta_from_url() - This basically performs a head request and returns the filetype and meta to store in S3. (Taken from sotoki. Maybe we shall consider having it in the scraperlib).
• is_optimizable() - Return true if the downloaded file is a video/image, i.e. able to optimize
• s3_credentials_ok() - Checks provided S3 credentials
• download_from_cache() - Check if a key is available in cache and matches the meta. If yes, it downloads it and returns true. Also checks the optimizer_version if --use-any-optimized-version is not given.
• upload_to_cache() - Uploads a file to cache at given key with given meta and optimizer_version if available. Returns True if success, otherwise False

Currently the following work -

• S3 cache download/upload
• Video convertion

The youtube download is not tested right now (will test soon)

[satyamtg]

Did those changes. Now we do not upload a file to S3 if any version identifier is not found in headers. Question is, shall we ignore the cyclic imports pointed out by codefactor (we can fix that with #57 )?

[satyamtg]

Have done those suggested changes and rebased on master. Have also bumped kiwix_storagelib version in requirements to use the new function.