Link validation

**Describe the feature**
In some cases, we might want to validate certain links before allowing the user to post it. So, we want a feature allowing us to define a certain links (maybe along some other information about those links), prevent them from being posted directly (maybe per certain role), notifies the moderators who would be able to validate it and allow or disallow posting of the link (could be simply re-sending the message through grace).

A good example of this are google forms. We don't necessarily want to ban those links but we want to validate that not personal information is asked in the form. So, we would probably want to prevent user from directly posting `https://docs.google.com/forms/...`.

Along this feature, we could probably add a auto deny feature. In other words, some links would not require moderators' attention and instead be denied automatically (if we caught a fishing link).

PS. This doesn't happen often so it's not necessarily a priority.