[C2] Calendar event update modal improvements

[ywarnier]

The current edition page for a (global) calendar event has several display issues:

• The color selector is not set by default on the color of the event (it always shows red by default, even if the color of the event is another color)
• "My invitationsSubscriptions" are 2 different buttons stuck together, which is very counterintuitive. Should appear as real buttons or tabs or anything else that could make sense
• The class selector in "Subscriptions" says "Social group", while the context points more to classes, so it should say "Class", not "Social group"