Allow welcome moderation for self-hosted

Applies to self-hosted /e/ Cloud.

Summary

Actually, anyone can register an account on a self-hosted, using welcome.

Description

What is the current behavior?

Anyone can reach welcome.$DOMAIN and create account(s), including bad guys.

What is the improved behavior?

Self-hosted administrators should be given the choice to moderate (accept/refuse) account(s) creation.

What does it bring?

Mostly, security against bad use of self-hosted.

Examples

Account creation is put on hold.
Administrator could refuse a illegitimate attempt, or accept if OK.

Validation

After registering a new account using welcome, administrator may get an email with Accept/Refuse links.
If she/he goes to the Accept link, account creation goes his way.

Checklist

• [ ] License
• Translations
• Documentation
• Test plan