Add a version attribute to templates

We store our custom templates in our git repo. With the repo as the canonical source, we fan out from there and import into our various Zabbix servers.

In order to more easily track what servers have what versions of templates, it would be nice if there was a version string tag for templates that is settable and viewable in the UI. It should show up as a column in the template list and on the property page for the template itself.

I suppose I could use a macro for this, but that seems a bit janky.