Fix missing user column in task table

[thomaslow]

The optional column "last editing user" is not shown in the task list when selecting it from the column configuration drop-down menu. This pull request fixes this problem. The column "last editing user" is now available.

[henning-gerhardt]

Your change looks strange for me: every around your code change is referring to processingUser and not to lastEditingUser except the column heading. If the lastEditingUser should be displayed then should not everything (user information, column id name, ...) used from lastEditingUser or named by this?

[solth]

@henning-gerhardt this is due to an unfortunate difference between the task property processingUser and the database entry task.lastEditingUser in the listcolumn table, which controlls whether the processingUser of the task should be displayed or not.

Thus this pull request indeed fixes the described problem.
An alternative solution would be to change the title value of the database entry from task.lastEditingUser to task.processingUser using a migration file in order to adjust bean property and list column flag.

[henning-gerhardt]

Thank you @solth for the clarification but reading the code without this information is confusing and I will maybe not the last user which gets confused.

Edit: I'm not saying that should be changed in this pull request but maybe in a different pull request.