Provide signed MD5 checksum on Bitbucket Server download page

Issue Summary

Currently the md5 checksum for Bitbucket Server downloads is unavailable for any releases past 5.7.0

The previous ticket for this request (BSERV-2480) was closed with the work around of being able to retrieve the md5 checksum by appending '.md5', but beginning with release 5.7.1 you can no longer retrieve the checksum in this way, and instead receive the following error - likely because the file is missing:

Steps to Reproduce

1. Attempt to visit https://product-downloads.atlassian.com/software/stash/downloads/atlassian-bitbucket-5.7.0-x64.bin.md5 
2. The md5 checksum is downloaded, and can be opened in a text editor for review
3. Attempt to visit https://product-downloads.atlassian.com/software/stash/downloads/atlassian-bitbucket-5.13.0-x64.bin.md5 

Expected Results

The md5 checksum for the 5.13.0 installer is downloaded, and the contents can be reviewed in a text editor

Actual Results

No file is downloaded, and you receive the following error:

Workaround

Atlassian Support is capable of providing a checksum for a given Bitbucket Server installation file on customer request