Export gist to personal GitHub account

[pathmapper]

Currently the export to Gist is creating an anonymous gist.

If a user is logged into GitHub, the gist would be created in the GitHub account.

Maybe the Login with GitHub functionality can be taken from #155 (comment) ?

Benefits:

• Directly creating a gist in the own GitHub account. Currently you have to fork the anonymous gist to do so.
• Ability to delete gists not needed/wanted anymore from the web browser. Currently you have to contact the GitHub support to delete an anonymous gist.

[orangemug]

I like the idea, probably on hold until I finish the GitHub integration in issue #155.

One thing we can do now is mock up a UI. The way I see it the export modal would need to account for

• anonymous (public/private)
• user (public/private)

It'd also be nice to tidy up the export modal in general. For example at the moment we also ask for mapbox/openmaptiles keys on export. If you're using the fallback keys, see #230 (comment), I don't think it's obvious why you're being asked for these keys.

I think a redesign of that modal would be worth considering as a part of this issue.

[pathmapper]

anonymous (public/private)

Not needed anymore in the future, see #269.