adding support for syndetics plus over SSL

It may look like this commit introduces backward-incompatibility by getting rid of the [Syndetics] 'url' setting, however, the code that called on that setting got buggy in the migration to vufind2 so it would never have worked for anyone. Instead of adding another 'url' setting, and since the base ssl url is the same for regular syndetics and syndetics plus, I replaced 'url' with 'use_ssl'. If 'use_ssl' is not set then we use the non-ssl urls that were also our prior default values.