Implement share button

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Add a "share" button in addition to the "save" button in the app bar for browsers that support Navigator.share() to allow users to directly share PDFs from Doclight to other apps. Checking to see if navigator.share exists should be sufficient for feature detection.

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Okay so it turns out you can only share files on Chromium and you can only share certain types of files (not including PDFs) so this probably isn't high priority. But implementing it proactively wouldn't be a bad idea in case a browser ever lets you share PDFs.

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Found out that navigator.canShare() is totally fine with telling you that you can share PDFs even though you obviously can't and it errors every time you try. Since I don't want to have a button that errors every time, I won't be implementing the share button until there's at least one browser where it would work.