1232: Create github release

[f1sh1918]

Short description

As a dev i want that after successful delivery a github release with release notes will be created.

Proposed changes

• add notify_release job to create a release, generate release notes, upload artifacts and send mattermost notification to release channel

Resolved issues

digitalfabrik/app-toolbelt#13 (merged)
resolves #1232, resolves #1319, resolves #1331, resolves #1165

Testing (dev only)

1. Important deactivate delivery tasks in deliver_beta_native(!!!)
2. Push your commit

 curl -X POST https://circleci.com/api/v2/project/github/digitalfabrik/entitlementcard/pipeline \
  --header "Circle-Token: <"your token">" \
  --header "content-type: application/json" \
  --data '{"branch":"1232-create-github-release", "parameters":{"run_delivery_beta_native":true, "run_commit":false}}'

[ztefanie]

Nice PR. 👍