[feature/crypto] Add method for obtaining the hash settings only

This is needed for combined hashing of passwords.

PHPBB3-11610
[feature/crypto] Use correct prefix while generating salt in salted_md5

PHPBB3-11610
[feature/crypto] Pass phpbb_container to crypto helper

PHPBB3-11610
[feature/crypto] Fix minor code limitations in crypto helper

These limitations caused it to only allow a specific input to combined
hashes.

PHPBB3-11610
[feature/crypto] Add function for obtaining only the hash to crypto helper

This is also needed for combined hashing of passwords.

PHPBB3-11610
[feature/crypto] Add combined hashing method to crypto helper

This will allow to hash a previously hashed password with another hashing
method, i.e. as upgrade path from phpBB 3.0 to 3.1.

PHPBB3-11610
[feature/crypto] Add tests for combined hashing of passwords

PHPBB3-11610
[feature/crypto] Make method get_hashing_public_algorithm public again

Combined hashing requires it for checking the supplied password hash for
its hashing algorithm.

PHPBB3-11610