Add a template provider for Scheduler jobs

It would be nice to use the Templeate Provider feature to create new Scheduler jobs. I propose to make it available only in the Scheduler space but to be able to create jobs in any location, since there are extensions that could need one, in their location.
At the same time, I think we can get rid of the "Create a new job" form and add a mention in the page description/content to use the "Create" button.

For this improvement, we also need to get rid of the jobName property from the XWiki.SchedulerJobClass and to rely on page name and page title.
To investigate if a migration script is needed, but it looks it would not be needed.