[Security/Http/RememberMe] PersistentTokenBasedRememberMeServices bugfix

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass? | yes
| License       | MIT

The database and debug layer cannot handle raw random strings. It may contain invalid ut8 characters and whatnot. So, in order to avoid a lot of database bugs, we must base64_encode the random strings.