Subsequent captures, UI updates and bugfixes

Once again, many of the changes in this release wouldn't have been possible without the support of Internews and the BASICS Project. And we would also like to thank Credit Agricole and @FafnerKeyZee for the continuous bug reports!

The main new feature of this release is the possibility to capture URLs present in a capture you already made, all that while keeping the context your browser was in (cookies, user-agent, referer) in the subsequent captures. It is especially useful when the page you're landing on expects the user to click on a link in order to load the content, the website checks the referer and/or cookies, and bounce you if you're not presenting the right session.
This feature will be further extended in the upcoming releases to allow other types of requests (POST), and let the user choose the link(s) to captures from the screenshot of the page itself.

This release also adds a new background indexer so the captures queued with the API are automatically cached even if they are never opened in the browser.

And there are the usual bunch of bugfixes, improvements and dependencies upgrades. And we also require python 3.8+.

You can also search for hostnames, URLs, hashes, and cookies names from the /search entry point. This entry point is not listed yet in the documentation, but it will be added soon.