Create a new "configpage" hook for those modules that need to inject anything in the configuration page.

This allows us to change the signature of the hook, so that we pass it the \SimpleSAML\XHTML\Template object and then the hook can not only add data to be passed to the twig template, but also add its own translation domain. This was needed because many modules were putting their translations inside "core", and when moving those to the modules themselves, the new translation system cannot perform the translation because only the translations for the current module in use are loaded (in the config page, that's the core module). Therefore, we needed a way to get the Localization instance from the template, and a way to pass that to the hook so that the hook can register the translations for its own module.

Translations for preprodwarning replacement

This allows more graceful fallback for untranslated languages.
Also, use `raw` filter to keep the embedded HTML working.

Show custom rejection message to unauthorised users

Rewrite Crypto::passwordHash() to use BCRYPT instead of unsafe algorithms

Closes: #974

Deprecate preprodwarning

It does not seem like something that is normal so a warning seems in
place. However, forbidding it does not add much value since we accept
unsollicited responses (from any idp) anyway.

Closes: #975

Create a setting for the allowed assertion offset

This uses the translations from the preprodwarning module,
and can be removed in 2.0 (together with legacy templates).

All the old LC_MESSAGES content from the old
preprodwarning module will have to be added...

Update OpenSSL RSA bit length in docs

Change documentation from recommending 2048 bit keys to using 3072 bit
keys.

Given that we're recommending people generate ten-year keys, 2048 bit
keys are probably a bit short. Almost all commercial certificate
authorities now recommend 4096 bit keys, and eduGAIN requires 3072 bit
keys for new federations.

This change aligns the SimpleSAMLphp documentation with the lower
eduGAIN requirement, since that still meets most standards bodies
recommendations for 2028 (ten years from now). cf
https://www.keylength.com/

This reverts commit 9e98109b.