Lost schema of S3 URL.

[fukumoto-ts]

Since v2.5.0, S3 URL schema is now lost.

This is caused by a change in this PR. #1762 (#1761)

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It would be good to change it like this.

Now:
https://github.com/blademainer/codimd/blob/7b1e4e3ddb2ef07d1170dcd819986252d5758596/lib/imageRouter/s3.js#L54

s3Endpoint = `s3-${config.s3.region}.amazonaws.com`

After:

s3Endpoint = `https://s3-${config.s3.region}.amazonaws.com`