Generated SPDX headers

[MauritsR]

Background

With Amsterdam#1509, copyright and license headers were added to the signals-frontend codebase. Currently, the headers are added or updated to reflect the latest changes/authors with each PR.

Issue

Adding and updating the headers to ensure correctness is a manual task. Because of this, it might be forgotten or not updated correctly.

Solution

A script checking for at least the presence and (if possible) correctness of the SPDX license headers. If desired, the script can be triggered as a pre-commit hook.