ICAL export feature

[Kartones]

WIP at branch https://github.com/Kartones/flask-calendar/tree/icalendar-exporter

Support exporting events into ICAL format.

Rough TODOs:

• [x] Each calendar should be published to a private url based file, so can easily be consumed by ICAL-capable applications/services (IIRC this branch already does it)
• [x] ICAL file should be updated upon saving any change to a calendar (IIRC this branch already does it)
• [x] Normal, non-repeating tasks should export basic fields: Date, time when starts (00:00 if all day), time when ends (for now, either one hour later than it started or 23:59 if all day), title as summary, details as description
• [ ] Repeating tasks support. Probably create Up to N months ahead of tasks (maybe configurable via settings? with a default of lets say 6 months). A task can repeat weekly per week day, monthly per month day or monthly on the first week day specified (e.g. "first monday of the month").

[davidag]

This feature looks really useful, let me ask you a couple questions:

1. I wonder what's the reason to code your own ICAL generator, instead of using icalendar (for example)?
2. Could you add a TODO list in this issue to help me figure out how can I best work on it? Or maybe just mention what's the next step :+1:

Thank you!!

[Kartones]

1. Double reason: On one side, I wanted to learn about the icalendar format, so (second reason) I didn't even searched if there were existing libraries. Now that I peeked a bit about the format I wouldn't mind adding an existing library like the one you mentioned :)
2. Ok, I'll try to detail it a bit more, although I dropped my initial idea of using it so I'm not totally sure about functionalities it should have. Open to suggestions :)