The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
Each job compiler script now must define a `compileJob(job)` function,
which will be called by Flamenco when necessary.
This makes it possible to run the script without a job, and get other
exported symbols from it, such as metadata about which settings its job
type needs/exposes.
