Simplify cron and allow to run at fixed time
The cron definition is complex. It does not allow to be sure when a task will be run. It has unused feature like number calls, repeat etc. It can be simplified for #2286 (closed) and #6117 (closed).
And it is not sure that it is 100% safe (string define model, function and arguments).
So I propose to simplify it by:
- remove unused features
- allow to set the minute, hour and day (depending on the frequency)
- use a selection field to define all possible "jobs"
- remove arguments (all current job have no arguments)