[FEATURE] ClamAV/VirusTotal waiting page

**Plugin(s)**
ClamAV and VirusTotal

**What's needed and why ?**
With a high number and/or big uploaded files, the ClamAV/VirusTotal scan can take some time. When that's the case, we need a way to do it async and display a "waiting page" to the user until scan is finished.

We can use the `ASYNC_SCAN` setting with a `yes` or `no` value.
