Configuration to mandate a minimum bit length of paste slugs

[ChrisLovering]

Currently by default a paste slug is auto-growing, making it easy to enumerate all pastes if you wish to do so.

In the case that people hosting this service don't want this to be possible, we should add a configuration option to enforce a minimum bit length on slugs.

This could be done by making hash_create (or slug_create, either one) do min(selected, configuration.xxx) or such, that way a lower limit could always be imposed.