[Security] Outsource all the BCrypt heavy lifting to a library

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

The [BCrypt bundle](https://github.com/elnur/ElnurBlowfishPasswordEncoderBundle) is already using the library.

This is a working implementation of #7247