Replace mcrypt with OpenSSL about hootdraft

[mattheworres]

This was actually way easier than I at one point thought, and I erroneously dove down the rabbit hole of completely re-doing the auth scheme in the app (which I've come to the conclusion will require moving off of Silex, and onto Symfony 4 - a future item, but a large refactor). Instead, just replace the mcrypt method and call it a day. Old salts generated with mcrypt are OK, the primary concern here is not one of security, but rather platform compatibility. PHP7.2 servers won't be able to have mcrypt installed, but OpenSSL comes by default.