Allow creation of calendar events that span multiple days

[fmartingr]

#discussion

Due to the nature of the reminders and use cases of the plugin we only allow creating events on the same day with custom start/end times.

Acceptance criteria

• Discuss wether allowing events than span multiple days makes sense.
• Determine how the UX of the create event modal should look like.
• Determine the constraints with the new parameters in place (don't allow event's in the past, control that end datetime is not before start datetime, etc).
• Document findings in a new issue to handle the task.

Related tasks

• Switch time inputs to input[type="time"] mattermost-plugin-google-calendar#52