Add a handler for the OpenAPI `taskOutputProduced` operation, and an
image thumbnailing goroutine.
The queue of images to process + the function to handle queued images
is managed by `last_rendered.LastRenderedProcessor`. This queue currently
simply allows 3 requests; this should be improved such that it keeps
track of the job IDs as well, as with the current approach a spammy job
can starve the updates from a more calm job.
`persistence.Model` contains the common database fields for most model
structs. It is a copy of `gorm.Model`, but without the `DeletedAt`
field (which triggers Gorm's soft deletion).
Soft deletion is not used by Flamenco. If it ever becomes necessary to
support soft-deletion, see https://gorm.io/docs/delete.html#Soft-Delete
This introduces some more conceptual changes to Shaman. The most important
one is that there is no longer a "checkout ID", but a "checkout path".
The Shaman client can request any subpath of the checkout directory,
so that it can handle things like project- or scene-specific prefixes.
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.
The build chain got a bit confused when doing things from scratch, as
`test_support.go` was used in the non-test builds. Renaming it to
`support_test.go` was the easiest way to avoid that.
