Add new ArkindexFeature values for export workers

https://redmine.teklia.com/issues/7497

Two new ArkindexFeature values should be added to support the two existing export workers, ExportPDF and ExportPageXML.

Unless there already is a tag on the export worker repo, you can use registry.gitlab.teklia.com/arkindex/workers/export:0.1.0 as the image for both workers in system_workers.yml, with the two separate commands as defined in .arkindex.yml.

The arkindex update_system_workers command also needs to be updated to support using the same Docker image for multiple ArkindexFeature with different commands. The current behavior would assign one worker version to one of the two exports, then immediately reassign it to the other export.

To handle this, the existing query to look for an existing worker version should also filter by configuration__docker__command. When no command is set for this feature, it should filter by __isnull=True to exclude any versions that have a custom command (and thus would not work properly). When a command is set, it filters normally with a =.