feat: Django command to generate/export a Vapid keypair

[David-Guillot]

Hi @safwanrahman ,

I found it weird to rely on an external service to generate a Vapid key pair, which is required to send Webpush notifications to most users. So I dug into py_vapid and cryptography stuff to understand how to generate and export the keys under the right format. So here it is, a nice Django command to make this Django app easier to use.

Let me know what you think,

[safwanrahman]

Very interesting. I like this idea. r+