dehacked: Remove custom messages for secret levels.

The C5TEXT and C6TEXT messages had custom messages for particular
levels that are no longer present in those slots now that the FreeDM
level order has changed. Change these so that they just match the
message shown for the other story screens instead.
dehacked: Use https: version of website link.

Github now provides encrypted https: access to its github.io pages.
Use the https: equivalent URL instead of the unencrypted http:
version. Update the CREDIT screen text to do the same.