Refactor Post IDs

[VincentRPS]

Generate a globally unique post ID using the following data combined into a value hashed with xxh. The hash made up of the following values separated by a zero-width unicode space:

• Permanent user id
• A random assortment of exactly 16 characters A-Za-z1-9, allowing a possible amount of 61^16 unique combinations. This is set by the server of the user id.
• A Unix timestamp, matching with the Posts original creation ts, not index ts.
• A signature of the previous two combined with no separator.