Add cron-like scheduler to Friend UI with REST API

[xet7]

Hi,
Wekan will sometime have IFTTT like here at screenshot, it's in development:
wekan/wekan#1160 (comment)

For Friend, cron-like scheduler would have:

• UI, where times to run can be added and deleted
• REST API, so with Wekan IFTTT I can setup cron time, and at that time scheduler could call IFTTT job in Wekan
• Also, Friend Calendar could send notifications at scheduled time.

[titlestad]

Actually, we could expand the ageing DoIt app to do just this. It was developed in 2014-15, but then development stopped. I don't see a reason why we shouldn't revive it. But this is client side.

As for command scheduling in Friend Core, there is a system there, but I am not too familiar with it. Perhaps @stefkosdev could chime in here.