Allow operations to define the operation value

Currently the value seems hardcoded and is only displayed:
#565 (comment)

But e.g. for autotagging we would like to write the tags into that value.
So we either need a way to select a different template, or the modification is done on rendering side.
To me the later sounds more difficult?

E.g. from autotagging I'd expect a select2 dropdown, similar to the UserGroupMember ship check.