S3 Upload with CDN assumes path is persistent

Your Rocket.Chat version: (make sure you are running the latest)
0.32.0

The documentation for the S3 Bucket file storage is lacking (ill work on that later), and as a result some debugging and general trial and error was necessary to figure out how to get everything to come together.

1. What was very confusing during the debugging process was understanding that any file uploaded while a CDN url is present is "stuck" on that particular path. I was going back and fourth wondering why the web console wasn't showing a new 302 location. General problem here is that there doesn't seem to be a way to move previous files, they're stuck at whatever path they were originally uploaded under.

2. Show some kind of HTML5 template for the input boxes, as otherwise it's confusing to understand what it wants. The description actually says "CDN domain for downloads",

   

   but this isn't true, as the 302 redirect, if using a domain, just appends it to the current URI (missing a scheme) - it should probably at least default to //. This is the root problem, as I followed the instructions to put the domain and was stuck after testing a few files because of #1.

3. Warnings should be present and obvious to the admin that currently, there's no way to back out of a domain choice or storage choice without serious consequences. Tools to migrate data between services (S3, filesystem, GridFS) and domains are missing

No source file or test is named. Start by reviewing the S3 bucket file-storage and CDN configuration described in the issue, then reproduce the redirect and previously uploaded-file behavior with the reported settings. Done should include clarified configuration guidance and explicit warnings about changing storage or CDN domains, if those are confirmed as the intended scope.