Add switch to hide/show inactive templates

[solth]

Fixes #3311

Instead of removing the switch to create unreachable templates, as was proposed in the linked issue, this pull requests restores the - presumably - intended behaviour of filtering the list of templates available to create a new process for a project by the templates active values. In addition, a switch in the template list on the project page can be used to toggle whether inactive templates are shown in that list or not.