Add CATEGORIES property to events

[dantetemplar]

Describe the feature

Needed for one-zero-eight/website#194

Suggested solution

Unique categories for events (formerly, event-chains)

Additional context

No response

[ArtemSBulgakov]

Before implementing, test how CATEGORIES are handled on export to Google / Outlook / Apple calendars

[dantetemplar]

@ArtemSBulgakov handled as... nothing

[ArtemSBulgakov]

@dantetemplar Which property is used by Outlook to display colors?

[dantetemplar]

@ArtemSBulgakov CATEGORIES, but user need to configure it by hand in outlook

[ArtemSBulgakov]

@dantetemplar So, will our categories be added to user's categories list automatically?

[dantetemplar]

@dantetemplar Google just removes CATEGORIES property

[dantetemplar]

@ArtemSBulgakov As far as I know no categories propagated to any list in Outlook (and that's pity)