Add URL and ID fields to Wikipedia dataset
{'login': 'albertvillanova', 'id': 8515462, 'node_id': 'MDQ6VXNlcjg1MTU0NjI=', 'avatar_url': 'https://avatars.githubusercontent.com/u/8515462?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/albertvillanova', 'html_url': 'https://github.com/albertvillanova', 'followers_url': 'https://api.github.com/users/albertvillanova/followers', 'following_url': 'https://api.github.com/users/albertvillanova/following{/other_user}', 'gists_url': 'https://api.github.com/users/albertvillanova/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/albertvillanova/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/albertvillanova/subscriptions', 'organizations_url': 'https://api.github.com/users/albertvillanova/orgs', 'repos_url': 'https://api.github.com/users/albertvillanova/repos', 'events_url': 'https://api.github.com/users/albertvillanova/events{/privacy}', 'received_events_url': 'https://api.github.com/users/albertvillanova/received_events', 'type': 'User', 'site_admin': False}
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Do you think we have a dedicated branch for all the changes we want to do to wikipedia ? Then once everything looks good + we have preprocessed the main languages, we can merge it on the `master` branch Yes, @lhoestq, I agree with you.

I have just created the dedicated branch [`update-wikipedia`](https://github.com/huggingface/datasets/tree/update-wikipedia). We can merge every PR (once validated) to that branch; once all changes are merged to that branch, we could create the preprocessed datasets and then merge the branch to master.  @lhoestq I guess you approve this PR?
This PR adds the URL field, so that we conform to proper attribution, required by their license: provide credit to the authors by including a hyperlink (where possible) or URL to the page or pages you are re-using.

About the conversion from title to URL, I found that apart from replacing blanks with underscores, some other special character must also be percent-encoded (e.g. `"` to `%22`): https://meta.wikimedia.org/wiki/Help:URL

Therefore, I have finally used `urllib.parse.quote` function. This additionally percent-encodes non-ASCII characters, but Wikimedia docs say these are equivalent:
> For the other characters either the code or the character can be used in internal and external links, they are equivalent. The system does a conversion when needed.
> [[%C3%80_propos_de_M%C3%A9ta]]
> is rendered as [À_propos_de_Méta](https://meta.wikimedia.org/wiki/%C3%80_propos_de_M%C3%A9ta), almost like [À propos de Méta](https://meta.wikimedia.org/wiki/%C3%80_propos_de_M%C3%A9ta), which leads to this page on Meta with in the address bar the URL
> [http://meta.wikipedia.org/wiki/%C3%80_propos_de_M%C3%A9ta](https://meta.wikipedia.org/wiki/%C3%80_propos_de_M%C3%A9ta)
> while [http://meta.wikipedia.org/wiki/À_propos_de_Méta](https://meta.wikipedia.org/wiki/%C3%80_propos_de_M%C3%A9ta) leads to the same. 

Fix #3398.
CC: @geohci
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