Add changelog generation (#761)

* WIP Release notes generation and templates

* WIP Add new sections to the template

* WIP renaming and wip

* Fix runtime template

* Add doc, NO_CACHE and tweaking of the templates

* Renaming cl into cumulus to make room for the polkadot and substrate

* Fetch data from Substrate and Polkadot

* WIP convert bash script to ruby

* Convert to Ruby

* Fix host function delection

* Extract priority to a macro

* Fix misc changes

* Draft release workflow

* Fix runtime dir

* Add ENV to ignore runtimes

* Install tooling separately

* WIP troubleshooting - remove sudo

* Minor formatting fixes

* Fix workflow

* Add missing dep

* Linting

* Fix changelog script

* Add missing tera install

* Use absolute paths

* Fix path + cleanup

* Fix changelog generation

* Add missing pre-release ENV

* Fix rust version ENV

* Fix release notes path

* Fix output

* Fix runtime_dir for cumulus

* Fix ENV substitutions

* Fix styling

* Debugging

* Styling

* Fix call to fetch the runtime version

* Cleanup and doc

* Delete sample .env

* Update scripts/changelog/templates/change.md.tera

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Change XCM emoji marker for a 

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Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>