Add Instagram posting support

[doersino]

This is really more of a "this could be cool and is now finally possible" rather than "will implement ASAP" kind of issue, prompted by a conversation with @btutal.

After years of systematically shutting down third-party libraries and tools that utilize private posting APIs, Instagram has finally added official support for automated posting via its Graph API – but there's a few catches:

• It's only available for business accounts. No biggie, those are easily created.
• Standard Instagram image aspect ratio and format (i.e., only JPEG) rules apply. No biggie either, just configure ærialbot to emit matching images.
• For API access, the Instagram account needs to be connected to a Facebook page for the "business", which requires a Facebook account. Major hassle to set up. (It's 2021, who creates Facebook accounts anymore?)
• Posting works by specifying a URL where the image is publicly available, there's no direct upload method. Some image hosting services like good ol' Imgur offer relevant APIs, but this adds complexity and another failure point.

Here's a Python-based tool that implements automated image (and video) posting to Instagram via this API: https://github.com/killianmcshane/autogram – its README.md also lists the steps required to set things up.

With posting through Instagram's website rumored to be on its way, perhaps some of these limitations and annoyances will fall away in the not-too-distant future, so it might be beneficial to be patient.