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.

Add routing & dependency injection support for easier coding and testing

closes: #473

Twig will always use gettext, legacy will use legacy. Twig does not
work with legacy so new ui would break if you would not also set this
option. Instead, just switch the system in use based on the usenewui
config variable.

Bugfix/name id policy

Closes: #177

This is because some templates, e.g. errror, expect to be able to
include them. This fixes e.g. the No State error page.

Removed Subversion section, and added need for npm