Add encoding of generated URLs

[afullerx]

This PR attempts to resolve #3124 by encoding any special characters in URLs auto-generated from file names. This was done by adding urllib.parse.quote() to app.add_media_file() and app.add_static_file().

The only difficult question was whether or not to also encode caller-provided URLs. Since url_path must be supplied in unencoded form to work with the FastAPI decorator, there should be no risk of double-encoding. The only downside of encoding it is that if a user passes in an url_path that includes special characters and relies on the return value being the same unencoded string, it could break their code. 

Since this is a pretty narrow edge case, I opted for this behavior to keep the operation of the methods more consistent. If you'd rather go the more conservative route of just returning the url_path provided value unencoded, that's obviously a simple change. However, then the return value description would have to be more complicated to fully describe the behavior.

[falkoschindler]

Thanks for the pull request, @afullerx!

I wondered if we could only URL-encode the file.name. But as you mentioned, @self.get needs the unencoded path. So I guess your solution is pretty optimal.

Regarding the breaking change: I would call it a bug that the returned URL couldn't be used to access the file if it contained special characters, which is fixed with this PR. If you really need the original url_path, it makes little sense to rely on the return value.