StatusView: clear MSB on calculated conversation_id

StatusView: clear MSB on calculated conversation_id

This field seems to be a left-over from the StatusNet era.
If your application uses `pleroma.conversation_id`: this field is
deprecated.

It is currently stubbed instead by doing a CRC32 of the context, and
clearing the MSB to avoid overflow exceptions with signed integers on
the different clients using this field (Java/Kotlin code, mostly; see
Husky and probably other mobile clients.)

This should be removed in a future version of Pleroma. Pleroma-FE
currently depends on this field, as well.