A user should be able to export reminders

[TimoWolters]

The reminder calendar uses the Internet Calendar Standard and is compatible with most other calendar systems. A user should be able to export their reminders as ICS file and import the reminders in their calendar applications. Some may want to export all entries every time, other may want to export and import only new or updated reminder. The export function should allow for both. This requires an additional attribute on the Reminder-Model updatedAt that determines the last time a reminder was touched by the user. The last export time should also be saved.