Investigate VirusTotal API to sanitise links

[Libbum]

If a user spams malicious links we want to be able to detect this threat and kill it instantly, also triggering the user's suspension somehow. The VirusTotal API can return a report on URLs. If we give admins the possibility of using the community API, this may thwart this threat vector. Of course, this is an external resource, so may be undesirable in general or for a large swath of users. So it must be optional.