[FEATURE] Upload files to VT

**Plugin(s)**
VirusTotal

**What's needed and why ?**
We need to add new files on VT and wait for the results in case a file is not already uploaded. This will increase the security.

The setting `SCAN_MODE` could be used with the following values :

* `hash` : don't upload files to VT, only use hashes
* `upload` : upload files to VT if the file is not already present

The scan of a new file on VT can take some times so we need to display a "waiting page" for the user.