Refactor magnet link copy function

[drew2a]

The function for copying the magnet link has been refactored. The code now is a bit safier.

Fixes #8016

[drew2a]

Yes, it is more verbose, but that is not necessarily a bad thing. More lines of code do not always equate to worse code.

It is more verbose because:

1. The logic was extracted into the function create_magnet, which makes it possible to properly test it. The previous version did not allow this.
2. The line:

        trackers = [
            tk['url'] for tk in self.current_download['trackers'] if 'url' in tk and tk['url'] not in ['[DHT]', '[PeX]']
        ]

was rewritten to:

        invalid_urls = {'[DHT]', '[PeX]'}
        urls = (t.get('url') for t in trackers)
        valid_urls = [u for u in urls if u and u not in invalid_urls]

This makes it more understandable by splitting a large and complex expression into two expressions with clear purposes:

1. Mapping the sequence
2. Filtering the sequence

[drew2a]

@kozlovsky, thank you for the review. I've addressed most of your comments except for the verbosity. For the explanation, please see above.