[Account] Add verified concept

[mellelieuwes]

1. Rename "researcher" flag to "creator" in user stable
2. Remove "student" flag from users table
3. Remove "coordinator" flag from users table
4. Add verified_at timestamp in users table

Users can be:

• Creator or participant (based on creator flag)
• Verified ✅ or Unverified❓ (based on verified_at timestamp)

Context:

• Only Verified ✅ Creators can publish on Next. Unverified❓Creators can only preview, nice for educational purposes.
• Only Verified ✅ Creators can have licenses to software services (future, for the time being verified creators can use all services)
• Creator verification process (Know Your Customer) is not automated yet and is done outside of Next.
• Admins will be able to turn on the verification flag for a specific user (see #827)
• Participants will be able to change their account type to Unverified❓Creator in their profile (see #826). This will make the Projects menu item available for them.

[mellelieuwes]

[TjerkNan]

Created #866 for this but the feature itself is ok.