Two-way variable implementation in the job submission end-point. Where
Flamenco v2 did the variable replacement in the add-on, this has now
been moved to the Manager itself. The only thing the add-on needs to
pass is its platform, so that the right values can be recognised.
This also implements two-way replacement when tasks are handed out, such
that the `{jobs}` value gets replaced to a value suitable for the
Worker's platform as well.
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.
