[RCD-37] ChangeLog: Add placeholder links for all tickets merged in 2.0.0
[RCD-37] ChangeLog: Add placeholder links for all tickets merged in 2.0.0

- List of placeholders generated from merge commits in the git log.
- Updated links in existing ChangeLog entries.
- Filled in ChangeLog entries for DEVOPS tickets.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl