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.
