Ensure that license header in source files is always present

[nikku]

As a follow up to #1231 we should establish a mechanism that ensures that the license header is always present (also in new files) and stays up to date. We should come up with something we can roll out across our entire project sphere (~90 bpmn.io projects), this project and friends.

Approaches

Incomplete list of directions to take:

• Git hook (needs to be installed by all contributors to work)
• GitHub Check + Github Bot (how could a bot help / support us)?

Bot Tasks

Possible tasks a bot can help us to deal with:

• Reject new files without license headers
• Automatically add license headers (for internal PRs)
• File PRs to renew license year

[nikku]

Solution we went for:

• enforce license headers via eslint rule (camunda-licensed/mit)
• not adding a license will fail CI
• npm run lint -- --fix will add the appropriate licenses