Get CSS recursively from instance

[satyamtg]

This gets the CSS files recursively from the instance by getting the dependencies from the CSS imported within other CSS using the @import keyword. This fixes the fonts with the PHZH instance.

The following changes are introduced to fix #94:

• download_dependencies_from_css() is now responsible for reading and rewriting the CSS content
• It now downloads CSS files recursively
• Filenames are kept as original (in download_and_get_filename()) to avoid complications as far as possible. If a filename cannot be inferred from the URL, a new one is generated by hashlib.

[rgaudin]

Can you please add noqa comments to bypass the codefactor here as we're using md5 for hashes and not security?

[satyamtg]

Can you please add noqa comments to bypass the codefactor here as we're using md5 for hashes and not security?

Changed that to a purely non cryptographic thing, xxhash64