Add user controlled prefix option to randomly generated masks

[BobbyByrne]

A feature I really enjoy from SimpleLogin/Proton Pass is the partially randomized mask generator. You control the prefix of the email and the rest is random. Example of this in practice.

While on a site (we'll use Github as an example) when you generate a new mask, the prefix will default to the site's domain and then set a random N number of random characters after dot, then @mozmail.com. github.89wfer@mozmail.com would be the result. You can then modify the prefix of the mask if you'd like. I've attached a screenshot of this feature in the Proton Pass browser extension.