removing %T in favor of an additional variable add_timestamp

[Melkor333]

A simplification of the whole timestamp stuff would be to completely hide the logic from the user (and therefore also eliminate configuration errors) by having an additional flag like:

[snapshot."bookworm-main"]
mirror = "bookworm-main" 
add_timestamp = true

That would also mean we don't need to parse the snapshot name to see if it has timestamps.

Originally posted by @Melkor333 in #118 (comment)