Import library from external source

[aaannestad]

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Feature details

I'm currently following over 600 series in on Mangadex. Rather than hand-adding every one of those series, it would be lovely to be able to import them all automatically. Not being able to import makes it hard for me to want to switch to Houdoku.

(This might be better put in the extensions repo, but I figured it would be nice to be able to import from a variety of sources.)

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