Backports to 1.11.8-stable (#9144)

* parity-version: bump stable to 1.11.8

* ci: update version strings for snaps

* Be more graceful on Aura difficulty validation (#9164)

* Be more graceful on Aura difficulty validation

* test: rejects_step_backwards

* test: proposer_switching

* test: rejects_future_block

* test: reports_skipped

* test: verify_empty_seal_steps

* parity: fix UserDefaults json parser (#9189)

* parity: fix UserDefaults json parser

* parity: use serde_derive for UserDefaults

* parity: support deserialization of old UserDefault json format

* parity: make UserDefaults serde backwards compatible

* parity: tabify indentation in UserDefaults

* Fix bugfix hard fork logic (#9138)

* Fix bugfix hard fork logic

* Remove dustProtectionTransition from bugfix category

EIP-168 is not enabled by default

* Remove unnecessary 'static

* Disable per-sender limit for local transactions. (#9148)

* Disable per-sender limit for local transactions.

* Add a missing new line.

* rpc: fix is_major_importing sync state condition (#9112)

* rpc: fix is_major_importing sync state condition

* rpc: fix informant printout when waiting for peers

* fix verification in ethcore-sync collect_blocks (#9135)

* docker: update hub dockerfile (#9173)

* update Dockerfile for hub

update to Ubuntu Xenial 16.04
fix cmake version

* docker: fix tab indentation in hub dockerfile

* ethcore: update to parity-wasm 0.31

* rpc: fix broken merge