Create an AWS S3 bucket for visual regression testing

[Pomax]

We need a new bucket for visual regression testing, set to "entirely" public as it will be hosting files that need to be web-accessible. I would recommend something like foundation-mozilla-org-visual-ci so that it's nice and unique while also relatively concisely describing its role.

We'll also need to add an access key and secret to the repo secrets (AWS_ACCESS_KEY_FOR_VISUAL_CI and AWS_ACCESS_SECRET_FOR_CI should work) so that the visual CI task can correctly make use of the new bucket.

[tomusher]

This has been created. Details in Engagement Team 1password under 'foundation-mozilla-org-visual-ci S3 Bucket Owner'