Fix the Azure storage mappings double-slash issue permanently

[jshearer]

Currently, Azure storage mappings generate fragment files with a double slash in their names. This is caused by this line including a trailing /:

flow/crates/models/src/journals.rs

Line 110 in a2ee6f5

"azure-ad://{}/{}/{}/{}/",

We can't just remove that slash, as that would cause the Azure fragment storage logic to generate incorrect signed URLs. So:

• Fix the Gazette azure storage mapping logic to return correct signed URLs for fragments that either contain a single, or a double slash
• Once that's live, then update the above to remove the trailing slash and start generating fragments with out a double slash

This won't solve the problem of having existing fragment files out there with double slashes in them, but it should allow us to start writing correct filenames, while retaining backwards compatibility with the existing ones.