Update Thunderbird metadata and add a Screenshot

[freaktechnik]

Prepare the Thunderbird metadata for what will likely be released later on this year and is already available in Daily and Beta.

CC @clokep

Preview: https://pr1304--matrix-org-previews.netlify.app

[freaktechnik]

I think this broke the site, because it redirects from https://matrix.org/docs/projects/client/thunderbird to https://matrix.org/docs/projects/client/mozilla-thunderbird, which now no longer exists. I think the redirect is generated in https://github.com/matrix-org/matrix.org/blob/master/scripts/generate-redirects.js

[afranke]

I think the redirect is generated in https://github.com/matrix-org/matrix.org/blob/master/scripts/generate-redirects.js

No. This is a single use script that keeps track of all the redirects we had and is not run any more. The redirect is the page at gatsby/src/pages/docs/projects/client/thunderbird.js. You probably want to reverse the redirect (rename the file to prefix it with mozilla- and remove the mozilla- from the url parameter in the meta tag).