chore: fix s3 redirects with prefixes

[crazy-max]

follow-up #15151

found wrong redirects with @thaJeztah ported from s3 config to cloudfront lambda.

looks like in the s3 config some redirects just strip the key prefix and don't just redirect to /. this PR adds a new bool field strip to handle that behavior.

can be tested on labs:

$ curl -s -D - https://docs-labs.docker.com/v17.06/enterprise/ -o /dev/null
HTTP/2 301 
content-length: 0
server: CloudFront
date: Wed, 17 Aug 2022 16:18:58 GMT
location: /enterprise/
x-cache: LambdaGeneratedResponse from cloudfront
via: 1.1 89cec266da5afe1c0fd332f7f04e94e2.cloudfront.net (CloudFront)
x-amz-cf-pop: CDG3-C2
x-amz-cf-id: yjyoCPapgzOBKtOHWgmDUOTvXUhLkf1B2Vl_sWSnL36NmpnzRkf2kA==

$ curl -s -D - https://docs-labs.docker.com/ee/sqdsqdsqdsqds -o /dev/null
HTTP/2 301 
content-length: 0
server: CloudFront
date: Wed, 17 Aug 2022 16:20:14 GMT
location: /
x-cache: LambdaGeneratedResponse from cloudfront
via: 1.1 ef7ace463c0659c94b8e007b4dc9ae9a.cloudfront.net (CloudFront)
x-amz-cf-pop: CDG3-C2
x-amz-cf-id: iT3UxQIdBi1TiqP6imgPm3Jbicl4wYJdXQoTS7usqmS25RpBq-C9QA==

$ curl -s -D - https://docs-labs.docker.com/get-started/part2/ -o /dev/null
HTTP/2 301 
content-length: 0
server: CloudFront
date: Wed, 17 Aug 2022 16:19:30 GMT
location: /get-started/02_our_app/
x-cache: LambdaGeneratedResponse from cloudfront
via: 1.1 bfb83f89a06636844c21e465f3ea5ace.cloudfront.net (CloudFront)
x-amz-cf-pop: CDG3-C2
x-amz-cf-id: L8gGrTkItVxWT9mRVziKUP1OUPdU5UPUoAH2km56F_Wt8RLDbXtIGA==

also disable slack notif for labs env as it seems a bit too noisy.

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[thaJeztah]

LGTM, nice!