feat(exports): Dashboard / Insight exporting

[benjackwhite]

Problem

See #9759 for more info

Changes

• Adds a new ExportedAsset model responsible for managing (and currently storing) exported assets such as pdf / png and in theory csv
• Adds Buttons for exporting to Dashboards and Insights
• Utilises Selenium for server-side rendering of the appropriate view and exporting

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

How did you test this code?

Outstanding tasks

• Production-isation of Selenium related code - Docker support, pre-downloading of chromium, ensuring memory requirements work etc.
• Dedicated standalone page for exporting of Dashboards / Insights instead of hacky use of SharedDashboard page
• Ensure tracking isn't enabled for exporter agent
• Ideally an E2E test to ensure the whole printing of an image works reliably

Misc

Closes #9851
Also closes #9799

[pauldambra]

I haven't run it yet so apologies if there are some incorrect assumptions in comments

Overall this is really cool 💖

[mariusandra]

Gave it a first look, left some thoughts! Overall, 🤩.

While I got one export, I managed to get stuck in a state where the next ones didn't download. I'm not sure why, as I didn't really see any debug output.

This could be on my setup somehow as well, e.g. Chrome being flakey.

However it's a bit weird that the toast disappears for a while, and only later the export succeeds. Could we keep the toast around until either a success or failure is detected... or the user closes it themselves?

[benjackwhite]

Somewhere along the way it seems i really screwed up this branch. There are bunch of changed files that I didn't change... Need to figure out what happened and somehow revert

[pauldambra]

I had to update Chrome to 102 before it would try to export...

Even then it's failing for me (hopefully me being silly :))