[PATCH proxmox 0/2] Enable upload button for snippets

[LightDiscord]

Hi everyone,

I wanted to use cloud-init by only interacting with the Web UI on Proxmox.
In all the steps required to do this, we need to reference snippets when 
using the `cicustom` option. However, it's not possible to upload them
through the UI. It's currently only possible to delete them.

The following two patches attempts to enable the upload route on the API
and enable the upload button on the UI. I've tested both of them and it
seems to work with the tests I did (upload and download from URL small yaml 
files).

NOTE: Some discussions for this feature had concern about the hookscript
snippets, in this case I'm focusing only on cloud-init snippets so I allowed
only `.yml` and `.yaml` files. Some people were also talking about Ignition
maybe enable all extension in the future ?

Reference:

* https://bugzilla.proxmox.com/show_bug.cgi?id=2208

NOTE: It's the first time I'm using git send-email and trying to contribute
to multiple repositories at the same time. If the subject of the e-mails or
anything else is wrong, don't hesitate to send me a direct e-mail.