Feature/image export

[mikekucera]

General information

Associated issues: #110

Checklist

Author:

• [x] One or more reviewers have been assigned.
• [ ] Automated tests have been included in this pull request, if possible, for the new feature(s) or bug fix.
• [x] The associated GitHub issues are included (above).
• [x] Notes have been included (below).

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• [ ] All automated checks are passing (green check next to latest commit).
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Notes

Adds a section to the Share Button popup that allows the network to be exported to an image. Options for image format (png/jpg), size (small/medium/large) and the area of the network.

[mikekucera]

The only problem is that export as PNG gives me a grey background.

Actually the exported PNG has a transparent background. I'll fix that now.