feat: create example event when a user logs in for the first time

[st3iny]

• Resolves: Create example events calendar#6608

Summary

Create an example event when a user logs in for the first time. The feature can be toggled and the event can be customized by admins.

I also harmonized the design of both settings sections (default contact and event) according to feedback from our designers.

With default data

With custom event and contact

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required

[st3iny]

Design feedback

• Example Content --> Example content (no capital c)
• Subline in --main-text "Example content serves to showcase the features of Nextcloud. Default content is shipped with Nextcloud, and can be replaced by custom content"
• For contacts: "Add example contact to user's address book when they first log in"
• For calendar: "Add example event to user's calendar when they first log in"
• Below each, there is an ⬆️ Upload secondary button and a ↺ Reset to default tertiary button
• Add download buttons:
  • 👤 default_contact.vcf ⬇️ (styled like a list item) ⬆️ Upload (secondary)
  • 👤 custom_contact123asdf.vcf ⬇️ (styled like a list item) ⬆️ Upload (secondary) ↺ Reset to default (tertiary)
  • Width of the file should be fixed (e.g. 200px)

[hamza221]

Tested and code looks clean 🚀

[nimishavijay]

Looks really nice now! :)
Only 3 minor changes for the file:

• Remove the underline
• make the text regular
• Add like 4px space between the text and the download icon

Other than that it looks great :)

[st3iny]

• Both example content features should be enabled by default